Fundamentals of Clinical Illumination and LED Technology

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ErgoPractice News – November 2013

In the beginning, God created the heavens and earth. Then God said, “Let there be light” and then there was light. Lastly, God created man and woman and gave them wonderful eyes which can see everything God created. Without light, or without our eyes to perceive light, we cannot see. This paper will review the functions of the eye, the fundamentals of clinical illumination, and applications of these fundamentals for selecting the right illumination systems and improving illumination quality with existing systems. Properly designed and adjusted illumination systems correctly light our work while protecting our eyes from intense light or potentially harmful optical radiation.

Purpose and Requirements of Clinical Illumination Systems

The purpose of clinical illumination is to help clinicians clearly see anatomical features in a comfortable working posture. The question is how we recognize good illumination. Requirements of clinical illumination systems are different from those required for other fields of work, such as industrial work, but there are still many common requirements.

Requirements for good clinical illumination may not be exactly the same for all different clinical procedures, but there are several common requirements. These are:

  1. The direction of illumination should not force clinicians to work in poor working postures.
  2. Good illumination should help clinicians easily see the detail and color of their target during procedures. Different procedures may require different spectral distributions of light.
  3. Light intensity should be easily controllable to optimize the brightness of objects to maximize the visibility of anatomical features.
  4. Good illumination should not hurt users’ eyes. The eyes have self-defense mechanisms, but the light which is too intense and/or light with too much blue light may hurt your eyes. All clinicians should be aware that intense illumination may be very harmful to the eyes over time.

There is a great variety of lighting systems being sold to clinical professionals today. The intensity of these illumination systems varies from less than 10,000 lux to more than 160,000 lux, which is equivalent to the intensity of sunlight on a clear summer day. Because the reflectivity of anatomical objects varies greatly, so should light intensity. Note that the intensity of typical ceiling lights in an office is less than 1,000 lux. This means the intensity of your surgical overhead light can be almost 200 times brighter than your office light. We can see how this can become a danger if used incorrectly!

Measurements of Light: Lumen, Footcandle, and Lux

The measurement of illumination intensity uses photometric units (lumen) which indicate the human eyes’ response to light. The measurement of optical radiation power uses radiometric units (watt). There is a simple relationship between photometric flux (lumen) and radiometric flux (watt). For a monochromatic light of 555 nm, 1-watt radiometric power will be 683 lumens photometric intensity to eyes. For other wavelengths of w in nm, 1 watt will be 683 x P(w) lumens to eyes where P(w) is the response of eyes to the light of w nm. P(w) is called a photopic response of eyes as a function of visible light wavelength. We will further review the photopic response of the eyes later in this paper.

A lumen is a unit used to measure the total light output from a light source. A large portion of a light source’s lumen output may be useless if it goes in the wrong direction. Footcandle and Lux are units that indicate the density (or intensity) of light that falls on a surface. The footcandle is equal to one lumen per square foot and a Lux is equal to one lumen per one square meter. So one foot-candle is 10.76 lux. The brightness of objects depends on the reflectivity of the object times the intensity of light that falls on the object. Note that the term “brightness” in common language often refers to “intensity.” Objects which reflect more light, those shiny or lightly colored, need less intense illumination sources.

Styles of Clinical Illumination Systems and Light Sources

Clinical illumination systems can be classified into two styles according to their mounting methods: overhead lights and headlights. And headlights are classified into two types: traditional headlights connected to light source boxes through fiber optic cables or to wall plugs through electric cables and portable headlights operated with batteries. Clinical illumination systems have used a variety of light sources: halogen lamps, metal halide lamps, xenon lamps, and various kinds of LED chips.

Among thermal lights, xenon is the most popular because its spectral distribution is very uniform over the visible range of 400 nm to 700 nm. The spectral distribution of xenon light is very similar to sunlight, but it generates a lot of heat (Figure 1). LED lights consist of two spectral bands, a narrow blue band, and a wide green/yellow/red band (Figure 2). LED chips do not generate infrared lights, so there is no such heat problem with LED lights.

With the continued advancement of white light LED chips, portable LED headlights have become very popular for clinical applications. (We will review the design of LED headlights later in this paper.)

Clinical illumination systems are intended to help clinicians clearly see anatomical features during specific procedures. Designers of clinical illumination systems should not only consider the target to be illuminated and the light source, but also how one’s eyes perceive this light. The human eye should be considered an important part of any illumination system. Before we discuss the quality of illumination systems, let us review the functions of the eyes and how they respond to light.

The Function of the Eyes

The eye is an incredibly sophisticated imaging system which is able to react to light 400 nm to 700 nm (Figure 3). The eye (1) creates images of objects onto the retina, (2) recognizes color differentiation from objects, (3) controls the size of the iris to adjust the amount of light reaching the retina for both good image contrast and protection of eyes, and (4) absorbs UV radiation under 400 nm to protect the retina from harmful radiation.

The iris of the eye quickly adjusts to control the amount of light reflected from objects reaching the retina. Under bright illumination, the pupil diameter can quickly drop from 3 mm to less than 1 mm as the illumination level changes. So, in over-bright conditions, the eye will block more than 90% of the light from a bright object. When the iris can no longer block the incoming light from the bright object, your brain recognizes the object as too bright and you will not see details of the object due to glare.

What is the Optimum Pupil Size for the Best Visual Acuity?

The size of the pupil may open as large as 9 mm in darkness. This increased pupil diameter increases the eye’s ability to see detail, like a camera (Figure 4). However, perceived detail diminishes at around 1.5 to 2 mm because the human eye is not a perfect lens. Brighter illumination may improve the depth of field because the diameter of the eye decreases to limit incoming light, resulting in better visual acuity over a longer working range. At the optimum brightness of illumination, the pupil size will be about 1.5 mm. But if one’s eyes feel a bright sensation and see glare, the illumination may be too bright.

Adaptation of the Human Eye to Changes in Brightness and Optimum Background Illumination

The eye can adapt to changes in light by a factor of 100,000,000 (from starlight to sunlight). The basic mechanisms of our eyes are responsible for adapting to changes in brightness is quite complex and includes changes in pupil size, breaching/regeneration of photopigment, and the responses of photoreceptors (cones and rods).

The time to adapt from a dark-to-light environment is much quicker than with a light-to-dark environment. This means the brightness of our non-target area (background illumination) should be lower than the brightness of our target area. Studies show that to achieve the best visual comfort we should maintain a proper target-to-background brightness ratio. The target-to-background ratio, or T/B ratio, compares the brightness of our target or work area to the brightness of the area surrounding the work area. This ratio affects the visual acuity and comfort of the eyes. A ratio of 3:1 is recommended for the best visibility and eye comfort. The work site should be at least three times brighter than the background. However, with many clinical worksites, the background is even brighter than the work site, the exact opposite of what is recommended!

Spectral Response of the Eyes to Visible Wavelengths: Photopic Response

The eyes are sensitive to a small band of the electromagnetic spectrum (400 nm to 700 nm) (Figure 5). The sensitivity of the retina to bright light is maximized at 555 nm. Almost nothing is below 400 nm or above 700 nm (Figure 6).

The brightness of objects perceived by the eyes is not the actual brightness of objects because the eyes attempt to block light in excess of what the retina can handle. The brightness of objects is determined by the intensity of illumination times the reflectance of objects. For example, the reflectance of mirrors is over 90% and black paint is less than 5%. Brighter objects need less illumination than dark objects. Generally, dental procedures require less illumination than surgical procedures because the reflectivity of teeth is very high.

The recommended illumination for industrial work with low contrast objects (matte) and very small objects is about 15,000 lux. Paradoxically, the intensity of typical clinical illumination systems are significantly higher than recommended for industrial procedures!

By trying different intensity levels, clinicians may find the most comfortable intensity level for maximum visual acuity with less glare and greater eye comfort. And, as we’ve learned, this does not always come from making the environment or target brighter.

What is the Color Rendering Index (CRI)?

When we view objects under different light sources, we notice differences in the way that surfaces are color rendered. Measuring the color quality of light with our bare eyes is very difficult. How do we subjectively compare different light sources? One way is to use the rendering index (CRI). CRI is a relative comparison between a light source and a reference light source. The resulting metrics denote color rendering accuracy: 90 to 100 – excellent color rendering: 80 to 90 – good color rendering; 70 to 80 – moderate color rendering.

There are a few limitations to CRI. CRI cannot be used to compare the CRI of two light sources of dissimilar color temperatures. CRI is a simple scalar value, but the light is a rich space of hue, saturation, and brightness. No single measure reveals everything about the quality of a light source. Even light sources with dramatically different spectral power distributions can have identical CRI values, but they render colors in very different ways.

Some will compare two light sources using a supposedly neutral target such as a piece of white paper. However, this type of test does not take into account the actual use of the light. That is, illuminating a target!

Two lights that have the same appearance on a white piece of paper may look drastically different when cast on anatomical objects. For example, red tissue may appear dull under a white light composed of yellow and blue light. Under an equally white light, but composed of a continuous spectrum, red tissue will appear a true red. Therefore, if we are evaluating the color quality of a light for clinical applications we should test light sources on anatomical objects and not a “neutral” subject, like a piece of white paper.

What are Color Temperature (CT) and Correlated Color Temperature (CCT)?

Color temperature (CT) indicates how emitted light appears when we observe the light source itself (Figure 7). Color temperature is measured in Kelvin (K), a scale that starts at absolute zero (-273 degrees C). Yellowish/warm white colored light has a color temperature of about 2700K to 2800K. As Kelvin increases, the light will appear whiter. Light with a color temperature of 5000K or higher appears similar to daylight (bluish-white light). Traditional tungsten light bulbs use CT, but gas discharged light sources and LED lights instead use correlated color temperature (CCT).

Spectral distributions of gas discharged lights and LED lights are much different from the spectral distribution of the blackbody radiation. CCT is a computed number using the chromaticity chart to closely match the blackbody radiation curve (often called Planckian line) on the chromaticity chart (Figure 8).

Two different light sources with the same CCT will not give the same appearance. CT is not the same as the color rendering index. The CT only measures the light output and does not indicate how the light appears.

Behavior of Light

After light strikes an object, three things may occur: the light is reflected, absorbed, or transmitted. More often, some combination of two, or even all three effects occur. In addition, there are three types of reflective behaviors: (1) specular reflection by a surface such as of instruments or of liquids covering anatomical objects, (2) spread reflection from rough surfaces, and (3) diffused reflection from matte surfaces. Reflected light delivers information about the features of objects, but strong specular reflection creates glare. Also, the polarization of reflected light is different than its source. One’s eyes cannot detect the change of polarization naturally. That is, without polarizing filters.

What is Glare?

Glare is light that enters your eyes but actually reduces the ability to see our target. Glare ranges from discomforting to the eyes to completely washing out target details. Also, glare may reach patients’ eyes and cause discomfort. To minimize glare, both intensity and direction of illumination should be adjusted. Figure 9 illustrates the effect of glare due to the intense illumination. When illumination is too intense, it washes out details of the object. Glare on a target makes a clinician’s job much, much more difficult and, in some cases, simply impossible.

How do we Evaluate the Quality of Clinical Illumination Systems?

The purpose of illumination is to help clinicians better see anatomical features. With this, they are able to better perform procedures and achieve better outcomes. Properly designed illumination systems should have: (1) a uniform beam, (2) no changes of beam pattern within the clinician’s working distance range, (3) high detectability of anatomical features, and (4) no glare.

To test for a uniform beam and changes in beam pattern over a working distance range, one may use a white piece of paper. By moving the surface to-and-fro, one can see if the beam pattern stays solid and clear, or changes colors or patterns.

The rendering ability of a light should be tested by illuminating actual anatomical objects, and not a piece of white paper. A quality light should enhance the visibility of anatomical features. As discussed earlier, two white light sources, which have the same white appearance on a piece of white paper, may differently render colored objects if the lights result from different wavelength compositions.

Co-axial Illumination for Deep Body Cavities

In order to deliver light into deep body cavities, the direction of illumination needs to be aligned with the line of sight of operators. Co-axial illumination assures this. With co-axial illumination, the clinician will avoid shadows from cavity walls or the clinician’s hands and tools.

Co-axial illumination systems come in two types: (1) lights on headbands or (2) lights directly mounted to dental/surgical loupes. The traditional headband-mounted light is heavy and cumbersome and easily shifts away from the clinician’s line of sight. In contrast, a light mounted onto loupes will always stay in line with the clinician’s magnified view.

Overhead Lights Versus Headlights

Overhead lights are quite effective when they illuminate relatively flat objects, but they are much less effective to illuminate the inside of deep body cavities. This is because, as clinicians work over their patients, their heads and bodies invariably block lights from overhead sources (Figure 10).

In order for overhead light systems to illuminate cavities, they use multiple lights utilizing the reflective property of the light. Since the reflectivity of body cavities is very low, overhead lights are set extremely bright to try to deliver adequate light inside of deep body cavities. This is a serious violation of one of the illumination’s fundamental rules: background light should be less than the target’s illumination. Traditional overhead illumination systems imbalance the T/B ratio, resulting in stressed eyes and strong glare. This means a more difficult job for the entire team, resulting in poorer outcomes.

We can overcome the drawback of overhead lights by using co-axial illumination for target illumination (Figure 11). Headlights deliver light directly into the body cavity. Once the target area is completely illuminated by the co-axial headlight, the 3:1 T/B ratio can be easily achieved by reducing the brightness of the now supplementary overhead lights.

LED Headlights

Traditional headlights, using fiber optic cables with thermal light bulbs such as halogen, metal halide, and xenon, are being used for shadow-free images of anatomical objects inside the deep body cavities. However, these traditional technologies have several drawbacks. First, they are tethered to a source box via fragile glass fiber optic cables (Figure 11). These fiber optic cables are heavy and cumbersome, which contribute to neck-strain problems. Other drawbacks include short bulb life, cost of bulbs, a drop of light output, difficulty in controlling the intensity of illumination, and heat radiation.

Recent advances in LED technology have made it possible to provide ever greater light output and better color rendering with much less radiated heat. Since LED chips are extremely energy-efficient, LED headlights can be powered by a small lithium-ion battery pack. This makes LED headlights portable, and thus gives the clinician mobility as well (Figure 12). Since they do not need fiber optic cables, they are very lightweight. The life of LED chips is much longer than gas discharged bulbs: for example, the typical life of a xenon bulb is less than 1,000 hours, but the LED chips may last for more than 20,000 hours.

The spectral distribution of a thermal light is continuous (Figure 1), but the spectral distribution of a white LED light consists of two spectral bands: a blue spectral band and a green/yellow/red band (Figure 2). The control of the intensity of a xenon light source is mechanical and quite difficult, but the intensity control of an LED light source is easily done electronically. Also, LED chips can be customized with unique spectral distributions for specific purposes. Customized LED chips can be used to enhance certain anatomical features in ways not possible with traditional thermal light technology.

LED headlights are steadily replacing traditional headlights due to the advantages mentioned above.

LED headlights are classified into four types according to how they generate their beams: (1) single-lens optic type, (2) reflective optic type, (3) single-lens & reflective optic type, and (4) achromatic multi-lens optic type. Beam patterns generated by the first three methods are similar, but the single-lens optic type is most popular. SurgiTel has recently introduced an achromatic multi-lens optic type (patent protected) which generates superior beam patterns (Figure 13a).

Figure 13 b & c show beam patterns generated with single-lens optics that are not uniform and have separated blue light around the main beam. In addition, their beam patterns and color uniformity will fluctuate as working distance changes.

The color quality or pattern of the beam generated with SurgiTel’s patented achromatic multi-lens system is distinctly uniform and will not fluctuate with working distance changes. This is important as separated blue light is even more harmful to eyes than blue light mixed with other colors. It is important to minimize, or avoid any potential risks to our eyes as they cannot be replaced once damaged!

LED headlights may use neutral white LED chips, cool white LED chips, or extreme cool white LED chips. How can we select the best available LED headlight for clinical application? As we discussed, we can evaluate beam uniformity by illuminating a piece of white paper and we can evaluate the color rendering ability by illuminating anatomical objects. In general, if we want to see accurate colors of anatomical objects we should consider headlights using neutral white LED chips.

Figure 14 shows three images of a toy bear and a flag under different lights. Image “c,” using a popular headlight of a different brand, shows the object in a blue haze. Not only is overall color inaccurate, but the color in the central part of the beam is different from that in the edges. Image “a,” taken using SurgiTel’s neutral LED, shows both enhanced color and clarity.

In short, not only should one be aware of a quality beam and color rendering but also what type of color rendering is best for the intended work.

Summary of Illumination Fundamentals

In this paper, we have reviewed the fundamentals of illumination systems and how they work with the human eyes. If we understand the fundamentals of illumination, we may all improve the quality of illumination in all clinical settings to do better work and achieve better outcomes.

  1. The sole purpose of clinical illumination is to help clinicians clearly see the anatomical features of their work without promoting poor posture and damaging the eyes.
  2. The size of the eye’s pupil is open 8 to 9 mm in darkness and is reduced to less than 1 mm under a bright illumination. When the pupil size is about 1.5 to 2 mm, the maximum visual acuity is achieved.
  3. Dark-to-light adaptation of eyes is significantly quicker than light-to-dark adaptation. This means the background area brightness should be much less than the target area brightness. The recommended target to background ratio is larger than 3:1.
  4. The sensitivity of the eye’s retina to wavelengths is the maximum at 555 nm and almost zero below 400 nm and above 700 nm. Unfortunately, the eyes are not particularly sensitive to the two main anatomical colors (blue and red).
  5. Light with using a uniform color spectrum may not work best for all clinical procedures. By selectively reducing the power of the green wavelengths we can enhance blue and red colors. This also reduces glare. Note that SurgiTel offers Contrast Enhancement filters.
  6. Color rendering index (CRI) is a relative comparison between a light and a reference light source. Although CRI may be used to judge the quality of the source of the illumination, there are several limitations. To evaluate the achieved quality of illumination we must review how the light renders actual anatomical objects.
  7. Color temperature (CT) indicates how the light appears when we look directly at the illumination light source. Correlated color temperature (CCT) is used for non-black body radiations. The CCT is a computed number using the chromaticity chart to closely match the blackbody radiation line (often called Planckian line) on the chromaticity chart. A limitation is the CCT of a light does not predict the ability of that light to render the color of objects under the illumination.
  8. CRI and CT (or CCT) may not be primary factors in selecting clinical illumination systems. The best illumination for a clinical procedure is that which best shows the anatomical features of the target.
  9. Diffuse reflection is what delivers to the eyes information about objects. Specular reflection is glare, which reduces the contrast of anatomical features.
  10. Specular reflection (or glare) can be reduced by reducing the brightness of illumination and/or adjusting the direction of illumination. Note the specular reflected light can be completely blocked by employing two polarizing filters: one with the illuminating light and another in front of eyes.

Application of Fundamentals

Case #1:
I am a surgeon and use both overhead light and headlight. My headlight doesn’t seem very bright. How can I improve my illumination?

Suggestion:
Fundamentals #1 and #2 may be applied to this case. With that amount of target and background light in the room, your iris is most likely quite closed. You can increase the pupil size of your eyes from 1 mm to 1.5 mm by reducing the background brightness. With the correct 3:1 T/B ratio, you will perceive the light is 125% brighter because more light will be reaching your retina. The correct T/B ratio will also help your visual acuity. At this point, you may even feel your headlight is too bright at which point you may reduce the brightness of the headlight. For certain procedures, it may even be beneficial to turn off the overhead light.

Case #2:
My hospital plans to purchase new surgical overhead lights and headlights. Can the color rendering index (CRI) and color temperature (CT) or correlated CT (CCT) be used to evaluate the beam quality of illumination systems?

Suggestion:
Fundamentals #1, #6, #7 and #8 may be applied to this case. If you have a reference light you have preferred to use, you can use CRI and use its CT or CCT as a reference. Fundamental #1 is the most important in this case. Light with a lower CRI may be better to show anatomical features. Therefore the quality of illumination systems should be evaluated by illuminating specific anatomical objects.

Case #3:
I am a dentist who has been using SurgiTel’s micro-LED headlight. I have been using both overhead light and the headlight. How should I balance the intensity of the headlight and the overhead lights?

Suggestion:
Fundamentals #2 and #3 may be applied. The headlight will be your primary light when both overhead light and headlight are used. First you find the optimum intensity of your headlight to achieve the best contrast of images and then you adjust the intensity of the overhead lights for background lighting.

Adjustment Procedure:

  1. Turn off both headlight and overhead light.
  2. Turn on the headlight and increase the intensity of the headlight slowly until it looks bright.
  3. Decrease the intensity of the headlight slowly until you see the maximum detail in the target area. The optimum intensity of the headlight should be at less than maximum intensity. (Remember: the light-to-dark adaptation of eyes is slower than the dark-to-light adaptation.)
  4. Turn on the overhead light and adjust the intensity of the overhead light. Note that the brightness of the background is lower than in your work area. It should be noted that many clinicians no longer use overhead lights if the regular room lighting is bright enough to see their surrounding area easily.

The procedure listed above may be used by clinicians to achieve the optimum light balance resulting in maximized visual acuity and visual comfort.

Case #4:
I am a dental student and I am interested in purchasing an LED headlight. There are so many dental LED headlights available. How can I find the right LED headlight?

Suggestion:
Fundamental #1 is the place to start. Many manufacturers may try to impress you with their claims of the brightest headlights. But as these manufacturers work to maximize their LED intensity at the center of their beams, the results are a non-uniform beam with a hot-spot in the center.

Key evaluation factors are:

  • Beam uniformity of intensity as well as color
  • The adjustability of beam intensity
  • Color balance for the best color accuracy of teeth. Neutral white LED offers the best color accuracy and the safest to eyes. Note that strong blue light of cool LED headlights may be hazardous as the light is reflected back into your eye.
  • No change of beam patterns and color balance over the working range
  • Will glare bother the patient’s eyes?

Note: you may use a white paper to evaluate the beam uniformity, but you should not use a white paper to evaluate the color balance of the beam. For the evaluation of the color balance of the beam, you may use an anatomical object such as nails, teeth of a patient or your friend, and teeth model.

Case #5:
I am a cardiothoracic surgeon. I use both bright overhead lights and bright xenon headlights. I feel that there is a lot of glare and cannot see anatomical features well. Are there ways to reduce glare and to enhance anatomical features?

Suggestion:
Fundamentals #2, #3 and #5 may be applied to this case. First, you may reduce the brightness of the background by lowering the intensity of the overhead light and adjust the intensity of the headlight. In addition, we may alter spectral distribution to reduce the counterproductive light and enhance the details of certain anatomical features such as veins, arteries or nerves.

SurgiTel offers an image enhancement filter that can enhance blue and red colors by reducing the green wavelengths. In this case, the green wavelengths are counterproductive light-pollution reducing perceived detail. If two polarizing filters, one in front of the headlight and one in front of the eyes, are used properly we can choose to enhance either the skin surface or enhance the veins under the skin.

LED Technology and Future LED Illumination Systems

Compared to traditional thermal light sources LED chips are extremely efficient and do not generate heat. LED lights can be operated with small batteries you can clip to clothing. The life of LED chips is very long. The control of intensity is very effective because the intensity is electronically controlled. Color rendering can be customized with different LED chips. SurgiTel offers various LED headlight options for dental and surgical applications. One may expect that, with the continued development of LED technology, more innovative clinical illumination systems will be available in the future.

 


References:
  1. Optical Society of America, The eyes and vision, Handbook of Optics, Section 12, McGraw Hill, 1978
  2. Guth SK, Light and Comfort, Industrial Medicine Surgery 27: 570-4, 1958
  3. Koninklijke Philips Electronics N. V., Basics of light and lighting, 2008
  4. B. Mustafa Pulat, Fundamentals of Industrial Ergonomics, Chapter 9: 211, Waveland Press, 1996
  5. Illuminating Engineering Society, IES Lighting Handbook, 5th ed. IES, 1972

 


B. J. Chang PhD is President and Chief Scientist of General Scientific Corporation, Ann Arbor, Michigan.

From the late 1970s to late 1980s Dr. Chang lead the development of advanced head-up display systems for fighter jets such as F15E, A10 and F4. Over the last twenty years he has been extending the ergonomic principles used for military display system designs to clinical vision systems. This has lead to SurgiTel’s family of ergonomic loupes and illumination systems which prevent chronic neck and back pain.

Numerous patents (awarded and pending) have been applied to the design of SurgiTel’s loupes and illumination systems.

Jin Chang PhD
Ann Arbor MI
jchang@surgitel.com
1-800-959-0153