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  • Mikko Jokikokko

LED Headlights as Clinical Illumination: Questions Raised by Clinicians and Answers

July 28, 2021

Traditional illumination systems used thermal light sources such as halogen and xenon which have continuous spectral distributions, but white LED lights consist of two spectral bands: “Blue” spectral band and “Green-Red” spectral band. Figure 1 shows spectral distributions of four light sources: two thermal light sources and two LED light sources.¹ LED headlights as the clinical illumination should meet two basic requirements. The first requirement is to help users see details of anatomical features of teeth or tissues. The second requirement is to minimize the blue light hazard by minimizing blue lights entering onto the retina.

Figure 1: Spectral Distributions of Light Sources and Blue Light Hazard & Visual Efficiency (Photopic Response) Functions


Four Types of LED Headlights

Figure 2 shows the strength of blue spectral band over green-red spectral band of variable brand LED lights and color temperatures.² White LED lights available today can be classified into four types: Warm (3,000K CCT), Neutral (4,000K CCT), Cool (5,700K CCT), and Extreme Cool (6500K to 72,500K CCT). SurgiTel’s “Warm LED lights” have the least blue light hazard and offer good color accuracy. The peak power of the “Blue” spectral band of Warm LED lights is about 50% of the peak power of the “Green-Red” spectral band.

Figure 2: Relative Risk Factor by LED Blue Light for various brands of LED light. Ratios are taken from reflected light at 35cm. Kelvin color temperature was measured from direct viewing.


Basic Factors for Optimum Illumination

There are several factors that should be considered in setting up the optimum illumination for various clinical procedures – selecting the right LED lights for specific procedures and optimizing the brightness levels of headlights and overhead lights.

  1. Eyes’ Adaptation to Light: “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 3.³ This means that the illumination of overhead lights should be dimmer than the illumination of LED headlights, but for many clinical cases, the background illumination is too bright.

  2. Reflectivity: Teeth reflect more blue light than red tissues. Dental professionals face more blue light hazards.

  3. Color Accuracy: Cosmetic dentistry may require the color accuracy

  4. Enhancement of Certain Anatomical Features: Images of certain anatomical features may be enhanced for easy diagnosis.

  5. Use of Optical Filters: Absorption or reflection or polarizing filters can be used to enhance certain features of anatomical features.

If you are interested in fundamentals of clinical illumination and LED technology, you may review an article published with ErgoPractice News. ³

Questions and Answers

We are receiving various questions about LED headlights from clinicians. We will answer some of those questions which are the most common.


Question #1: Is there an ideal clinical LED headlight?

Answer: No. For dental procedures, neutral or warm LED lights are recommended. Neutral LED lights offer the best color accuracy and are safe for the eyes. If you want to minimize the blue light hazard, you can use warm LED lights which are similar to halogen light: good color accuracy and the safest for eyes. For surgical procedures, cool LED light may be a better choice.

Question #2: Are brighter lights better?

Answer: The definition of good illumination is neither too bright nor too dim. Eyes can adapt to the wide range of brightness by controlling light which is entering the eyes by adjusting the iris of the eyes (1mm to 3mm under the bright illumination). If you can control the overhead illumination to meet the recommended target to background ratio, by adjusting the brightness of background illumination, all LED lights available today are bright enough for most procedures. If you compare cool LED lights with warm or neutral LED lights, cool LED lights will make you feel brighter because these lights have a stronger blue light. Cool LED lights not only distort the color but are also harmful to the eyes.

Question #3: Which LED lights do you recommend for dental procedures?

Answers: Neutral or warm LED lights are recommended. SurgiTel neutral LED lights have a balanced spectral distribution and thus offer the best color accuracy and are relatively safer for eyes. If you want to minimize the blue light hazard, you may choose SurgiTel’s new warm LED lights which offer good color accuracy and are the safest for eyes as shown in figure 3.


Figure 3: Comparison of Neutral and Warm LED Light


Question #4: Which LED lights do you recommend for surgical procedures?

Answers: Cool LED lights are recommended for surgical applications because cool LED lights will enhance the image of veins & nerves. Because red tissues absorb most blue lights, surgeons will face less blue light hazards.

Question #5: I am a surgeon and feel that SurgiTel LED headlights are not bright enough for my surgical procedures. Do you have brighter LED headlights?

Answer: SurgiTel will continue to design new LED headlights with new features. If you reduce the brightness of the overhead illumination to meet the target to background ratio (3 to 1), you will find that our LED headlights become very bright. My personal opinion is that surgical overhead lights are too bright. Too bright background illumination will strain your eyes and dry tissues quickly. Surgeons who have been practicing with reduced overhead lights told me that their eyes are no longer strained.

Question #6: I am a dermatologist and want to see veins under the skin. Do you offer filters to help me see veins better?

Answer: SurgiTel offers contrast enhancement filters that enhance the image of veins and cross-polarization filters that block lights reflected from the skin surface, allowing lights scattered from veins under the skin. With cross-polarization filters, we can enhance the image of veins under skins or the image of skins. Figure 4 shows effect of filters on images.


Figure 4: Effects of Cross-Polarization Filters and Contrast Enhancement Filters


Question #7: Can the color rendering index (CRI) be used to evaluate the quality of LED headlights?

Answer: The color rendering index (CRI) was developed to evaluate traditional thermal light sources for architectural lighting by using sunlight as the reference. CRI is a relative comparison between a light source and a reference light source.

Using CRI for evaluating the quality of LED lights as the clinical illumination is not proper because lights with low CRI can show anatomical features better. LED lights as the clinical illumination should be evaluated by illuminating specific anatomical subjects.

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. 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 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 light for clinical applications, we should test light sources on anatomical objects and not a “neutral” subject, like a piece of white paper.

References

  1. Chang, BJ, “What They Didn’t Teach You About Dental Lights in Dental School!.” ErgoPractice News, May 2021

  2. Price R, “Spectral Distributions of Various Brand LED headlights.” Private Communication, May 2021

  3. Chang, BJ, “Fundamentals of Clinical Illumination and LED Technology.” ErgoPractice News, November 2013

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