Advances in SurgiTel Headlights: Design Considerations of LED Headlights for Color Accuracy and Eye Safety

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ErgoPractice News – March 2014

Brief History of SurgiTel’s Development of Headlights

In the early 1990s SurgiTel developed the first (maybe the first in the world) clinical portable headlight for dental and minor surgical procedures (Figure 1a). This was accomplished using miniaturized halogen light bulbs. Soon after, SurgiTel introduced loupe-mounted, lightweight fiber optic lights for surgical procedures (Figure 1b) which can be used with existing fiber optic light sources, such as xenon and halogen light sources. They were designed using lightweight optics which do not require a headband. SurgiTel’s loupe-mounted fiber optic headlights eliminated heavy headbands and eliminated the issue where headbands would slip out of alignment. The elimination of the weight of headbands has helped users work with less stress on their necks.

In the early 2000s SurgiTel started to develop portable headlights using custom LED chips. Today SurgiTel offers several styles of LED headlights (Figure 2: Micro/Mini Headlight and Surgical Headband Light). Recently, SurgiTel invented non-contact switching technology facilitated by a miniaturized proximity sensor. Using what we call the Eclipse Switch, a clinician can turn on/off LED headlights without touching ether light or battery pack. This feature is not only convenient but helps clinicians avoid the dangers of cross-contamination. Eclipse LED headlights will be introduced a few months after the first publication of this article.

Design Considerations of LED Headlights

LED lights are an artificial light source which is significantly different from traditional thermal lights (such as halogen lights). Artificial white light LED lights consist of two spectral bands: a blue band and a green band (Figure 3). Based on the balance of the blue peak versus the green peak, clinical LED lights can be classified into three types: neutral white, cool white, and extreme (blue enhanced) cool white. Realizing that LED lights may damage the retina of the eyes1-8, SurgiTel established two main goals for designing LED headlights: (1) Help clinicians see well-rendered anatomical features to help their work and (2) Minimize potential eye hazards.

To design LED headlights that help users see better and are safe to eyes, we first reviewed how eyes interact with lights. The human eye is an excellent optical device but sensitive to a small spectral band (400 nm to 700 nm) of electromagnetic radiation. The sensitivity of the retina to light is maximum (100%) at 555 nm. Conversely, retinal sensitivity is almost zero below 400 nm or above 700 nm (Figure 4). Eye sensitivity at the blue peak (about 440 nm to 450 nm) is less than 5% of the maximum. We need the adequate blue spectrum to accurately see the color of objects, but a strong blue light can damage cells of the retina, resulting in early-onset macular degeneration. The lower portion of the blue spectral band (under 450nm) of white LED lights does not contribute to color rendering significantly and can also be very harmful to the eyes.

How are Extreme Blue LEDs Dangerous to the Eyes?

Eye lenses accept and absorb UV radiation with wavelengths less than 400 nm. Thus, UV light less than 400 nm is blocked from the retina of the human eye. The eye lens does allow wavelengths from 400nm to 475nm (blue light) to pass to the retina (Figure 5). Exposure to white LED lights with a strong blue spectrum can cause cellular damage to the retina through photochemical action. It is well known that this type of blue light is toxic to the biology of the eye.

Since human eyes are very insensitive to blue light, but have a high transmittance of the blue spectrum, blue lights can easily reach the retina and damage cells. To block the harmful blue lights, orange protection filters or orange eyeglasses are recommended when dental curing lights are used. Curing lights emit the same blue spectral wavelengths as LED headlights. Despite the awareness of curing light safety, dental professionals have not been well warned about the risks of improperly designed LED headlights.

Why is Separated Blue Light More Dangerous than Blue Light Mixed with Green Light?

The eye limits excessive light from reaching the retina by adjusting the size of the pupil. If blue light is separated from green light, the pupil will stay open because the retina is very insensitive to input of the blue light. In a way, the eye is “unaware” that it is viewing a dangerous source of light and unable to protect itself. The sensitivity of eyes to blue light is less than 5% of that of green light. If blue light is not properly paired with green light, about 20 times more blue light reaches the retina (due to lack of pupil contraction). This is why LED headlights with color-dispersed beams (with a separated blue “ring” around the main central beam) are more hazardous. Figure 6 shows both an achromatic beam pattern generated by SurgiTel’s multi-lens achromatic optics (all wavelengths overlap, with no hot spots) versus dispersed beam patterns generated by traditional single-lens optics (blue wavelengths are separated in a “ring” around a central hot spot).

Design Requirements of SurgiTel Portable LED headlights

The main purpose of headlights is to help clinicians see the relevant anatomical features of their patients better, so clinicians can work better. Our investigation showed that neutral LED lights render objects better with most dental procedures, but cool LED lights can better enhance certain anatomical features during surgical procedures. As a result, SurgiTel decided to offer two light formations: Neutral and Cool white LED headlights. Note that the majority of LED headlights used by dental professionals today are with either cool or extreme cool white LED chips. These make customers feel their light is “bright” because the undesirable strong blue spectrum is easily transmitted to the retina and stimulates perception. SurgiTel’s investigation found that cool or extreme cool LED lights not only reduce the image contrast of anatomical objects (reducing visual aquity) but may also contribute to ocular hazards such as early-onset macular degeneration.

Reviewing various issues related to headlights, SurgiTel established their design requirements for beam-forming optical systems of headlights:

  1. Produce a uniform beam
  2. Maintain a consistent beam pattern (in both color and uniformity) even as working distance changes
  3. Produce an achromatic beam (all wavelengths need to be overlapped/perfectly mixed) with no separated blue light
  4. Must not generate a glare which may blind patients
  5. Must be lightweight
  6. Non-contact switching option which may avoid cross-contamination risks

We evaluated various beam-forming optical concepts using different optical elements. Most manufacturers default to using simple, beam-forming optical systems with simple lenses. These inexpensive, optical systems do not require precision alignments. Also, SurgiTel tried the single-lens approach as beam-forming optics but we abandoned this approach because it does not satisfy any of the first three, most important requirements.

Our earlier LED headlights used a reflective optical system which satisfies the first three requirements. However, this design has trouble satisfying requirement #4 (eliminating glare). We continued to investigate new optical designs and we later invented the multi-lens, achromatic optical system (patented). This design satisfied all requirements except for the last item. We introduced these next-generation LED headlights in 2011 (Figure 2). Figure 6 shows the beam patterns generated by traditional LED headlights using single-lens beam-forming optics versus SurgiTel’s LED headlights using patented multi-lens beam-forming optics.

CR Foundation evaluated various LED headlights and stated that SurgiTel’s Micro Neutral LED offers the best color accuracy among eight (8) small LED headlamps/headlights9. It was also found that SurgiTel’s Micro Neutral LED headlights have the lowest blue light level10.

SurgiTel has continued to make product improvements, including non-contact switching (requirement #7). We considered “wireless” approaches which have been employed by one manufacturer. But, we abandoned this approach because this requires adding more weight to the frame. Adding more weight not only makes loupes uncomfortable but also it violates some of the other basic ergonomic design rules. We also considered adding a push-button switch to our battery pack. Although that may help avoid cross-contamination, we decided to search for a better solution. As a result of continued research, we invented no-touch, Eclipse switching technology (patent-pending). We plan to introduce this new product within a few months. We will review the features of “No-Touch LED Headlights with Eclipse Switching Technology” in one of our future e-Newsletters, “ErgoPractice News.”

Questions and Suggestions

Question #1: How should the beam quality of LED headlights be evaluated?

Suggestion: Beam patterns, or the change of beam patterns (color and shape) as a function of the working distance, can be evaluated by illuminating a white paper and moving the light towards and away. Color accuracy should only be evaluated by illuminating an anatomical object such as your hand11.

Question #2: I purchased my LED headlight three years ago. How can I find the LED color type of my LED headlight?

Suggestion: You may contact the manufacturer of your LED light. But until SurgiTel started to offer the neutral LED option in 2011, all LED headlights featured either cool or extreme cool LED chips. If an anatomical object such as a hand is illuminated with a cool or extreme cool lights, the color of the object will be distorted11.

Question #3: I am a dental student and interested in purchasing a LED headlight. I know that SurgiTel Micro LED headlights have the best beam quality, but are the most expensive. Some of my classmates bought cheaper LED headlights. Why should I purchase SurgiTel Micro LED headlight?

Suggestion: First, I would like to explain why SurgiTel Micro LED headlights are more expensive. Other lights use simple beam-forming optical systems that use inexpensive single lenses and do not require precision alignment. SurgiTel lights use achromatic precision multi-lens optics (patented) which use several expensive precision lenses that require precision alignment. This is the reason the cost of SurgiTel LED lights is higher than other lights.

There are several important reasons you should consider SurgiTel lights. According to CR Foundation’s evaluation of all major LED headlights, SurgiTel neutral LED lights offer the best color accuracy and the most uniform beam without glare which can hurt/blind the eyes of patients9. Also, CR Foundation found that SurgiTel neutral Micro LED headlamps have the lowest blue light10.

Question #4: If certain types of LED illumination may be harmful to our eyes, why have schools not raised awareness about the potential risk of long-term eye damage?

Suggestion: The use of LED headlights have become very popular only within the last several years. So there are no established standards yet for regulating the safety of LED clinical illumination. Dr. Price of Delhousie University (Canada) is also bringing attention to the subject by posting a warning video online (click here to see video). Recently Dr. Stamatacos and Dr. Harrison (of University of Tennessee Health Science Center) developed a continuing education article12 on this topic (click to read abstract).

 


References:
  1. Rozannowska M, et Rozannowska M, et al. Blue-light induced singlet oxygen generation by retinal lipofuscin in on-polar medica. Free Radic Biol Med. 1998 May;24 (7-8): 1107-12
  2. Sparrow RJ, et al. The lipofuscin fluorophore A2E mediates blue-light induced damage to retinal pigmented epithelial cells, Invest Ophthalmology Vis Sci 2000; 41:1981-9
  3. Delcourt C, et al. Light exposures and the risk of age-related macular degeneration. Arch Ophthalmology 2001; 119:1463-8
  4. Sparrow RJ, et al. DNA is a target of the photodynamic effects elicited in A2E-laden RPE by blue-light illumination. Invest Ophthalmology Vis Sci 2003; 44:2245-51
  5. Algvere, PV, Marshall J, Seregard S. Age-related maculopathy and the impact of blue light hazard. Acta Ophthalmol Scand 2006; 84 (1): 4-15.
  6. Overview of LED Product Safety, A European Perspective, Photobiological Safety of Lamps, May 2009 Bentham Company: www.bentham.co.uk (technical guides/pdf)
  7. Optical safety of LED lighting. (CELMA) European Lamp Companies; 2011. “http://www.celma.org/archives/temp/CELMAELC_LED_WG(SM)011_ELC_CELMA_position_paper_optical_safety_LED_lighting_Final_1st_Edition_July2011.pdf”.
  8. Cruickshanks KJ, et al. Sunlight and 5-year incidence of early age-related maculopathy: the Beaver Dam Eye Study. Arch Opthalmology2011;119: 246-50
  9. Clinicians Report, Is your LED headlamps damaging your eyes?, A Publication of CR Foundation, March 2013, www.CliniciansReport.org
  10. Clinicians Report, Smaller and lighter LED headlamps, A Publication of CR Foundation, April 2012, www.CliniciansReport.org.
  11. Chang, BJ, Fundamentals of clinical illumination and LED technology, EgoPractice News, November 2013; www.surgitel.com (under “News”)
  12. Stamatacos, C and Harrison, JL, The potential ocular hazards of LED dental illumination applications, continuing Education Article – Exam #51, J Tennessee Dent. Assoc. (Publication date: Fall/Winter 2013, Expiration Date: Fall/Winter 2016)