Amber Czajkowski, Edmund Optics
Medical doctors and dentists make visual diagnoses almost every day. They may identify a mole that needs watching, or catch signs of gum disease. The unique spectral characteristics of biological tissue cause it to vary in appearance according to how it is illuminated. Hence, the quality of the lighting used for these inspections is crucial: Inadequate lighting can lead to a misdiagnosis or, worse, failure to notice observable discrepancies in the tissue. If accustomed to working under poor lighting conditions, doctors may not even realize that they are overlooking essential details.
Fortunately, scientists have studied a range of lighting conditions in great detail. They have identified those that allow for optimum viewing, as well as lighting effects that could induce visual errors. This information can greatly improve illumination for medicine and dentistry. By choosing filtered light sources or by integrating optical coatings, one can replace insufficient ambient lighting in an office or examination room with consistent illumination that maximizes viewing and perception.
Daylight is best
The unique spectral characteristics of biological tissue result from the way it intrinsically reflects, transmits, absorbs and scatters incident light. Studies show that these properties vary according to the type of tissue, and as a function of the position and depth within the tissue structure. By tailoring the spectral characteristics of the transmitting light source – for example, wavelength and output power – it is possible to selectively augment features within the tissue.
This target selectivity is crucial for certain optical or biomedical imaging applications, but what about general diagnostic viewing? Typically, during routine checkups and examinations, physicians and dentists are not examining a specific location but rather the general appearance of a cavity or body part. Of most interest is stratified squamous epithelial tissue, which covers external and internal body surfaces, functioning as the predominant skin structure and lining most body orifices and cavities. The tissue is quite thick and can exhibit chromatic variations ranging from outer skin hue to a deep-red vascular color. Because the spectral properties of this tissue type can differ drastically, the physician needs a broad-spectrum source that can accommodate a variety of viewing conditions.
Consistent and exceptional lighting conditions are particularly important while observing a body cavity or orifice where impeditive spectral effects are maximized. Most standard light sources – in particular, incandescent and fluorescent lighting – fail to provide the distribution of light optimum for viewing. The most sensible approach is to modify the source so that it maximizes photopic vision, or natural daylight conditions. This type of illumination enhances visual acuity by causing the pupils to contract, which reduces aberrations in the eye and increases depth of focus. Natural light also has color-rendering properties that reveal true and natural colors. Studies have shown that these illumination conditions are best achieved with a full-spectrum daylight-emulating source.
Similarly, metamerism can occur during medical or dental evaluations where color discernment is vital for proper diagnosis. This is especially true with the use of incandescent sources, which emit more in the red-wavelength region and are more apt to blend in with the vascular tissue. Thus, incandescent lighting can hide significant color discrepancies that could indicate abnormal tissue formation. On the other hand, a short-wavelength daylight source, transmitting in the blue, acts advantageously as a contrasting agent, allowing observation of abnormal tissue. For gynecological applications, this could mean the difference between noticing a region of precancerous dysplasia and missing it completely.
Although certain medical illumination devices are intended for specific procedures, they are not always designed with the optimum light source in mind. They may not take into account ambient lighting effects that can occur if the standard incandescent/fluorescent room lights are not completely dimmed during use. Stray light potentially can alter the device’s spectral color quality.
In the gynecological example mentioned above, for example, if the physician, who is trained to visually recognize abnormalities, does not see the problematic region initially, the tissue is never sampled. Therefore, the biopsy is useless, despite the various procedures, such as cell staining, available for making an accurate diagnosis. Using color-corrected daylight-emulating sources is a good idea for procedural devices as well as background lighting.
The same is true for dental practitioners, whose offices and dental chairs are frequently equipped with light sources emitting primarily in the red. This illumination provides no contrast leverage to the already red-color oral cavity. Incorporating full-spectrum sources would not only illuminate those hard-to-see features but also aid in several cosmetic dentistry applications, such as tooth-shade matching, which are subject to metameric effects.
Achieving the best lighting
Incandescent (filament-based) and fluorescent bulbs are the most prevalent types of lighting. Fluorescents, however, are notoriously bad for true color rendering because of their “spiked” spectrum. This characteristic limits the number of colors the eye can discern, resulting in a dull-looking object, so it is usually best to start with an incandescent source when trying to achieve a daylight spectrum.
Figure 1. These graphs use Planck’s law to compare the sun’s blackbody spectrum to that of an incandescent source.
Daylight illumination can be emulated with a full-spectrum light source that combines blackbody effects inherent in both sunlight and skylight. The blending of skylight, which has a very high apparent color temperature (20,000 K), with direct sunlight (5300 K) yields a “daylight” condition with a blackbody color temperature that exists at about 5500 K or higher. As illustrated in Figure 1, the peak radiance of the incandescent source is drastically shifted to higher wavelengths, giving a “warm” appearance to those objects illuminated by it versus their appearance with the “ideal” daylight source. Although this daylight condition is perceived as “natural” lighting outdoors, it is surprisingly bluish in appearance when translated to an indoor environment. Converting the visible spectrum (region between dashed lines) of the incandescent source to one that more accurately mimics natural daylight can be accomplished by filtering the source. This can be achieved with dichroics – thin-film coated substrates that are often used as color filters and that are subtractive or additive in nature. In general, a custom filter design can be generated such that the nominal incandescent source multiplied by the spectral curve of the filter will yield the desired daylight spectrum to be transmitted forward. Integrating the resulting filter into the initial source design yields the desired transmission and reflection characteristics for the spectrum.
Figure 2. Cold mirror coated ellipsoidal reflectors reflect “cool” light forward and reject heat from the unwanted spectrum.
Another way to attain the desired spectral distribution from an incandescent bulb is by applying a cold mirror coating to the reflector portion of the bulb. A cold mirror acts as a dichroic that reflects the cool/visible light and transmits the unwanted hotter/IR light. The coating can be tailored to selectively reduce the redder portions of the visible spectrum and transmit them through the reflector and out the back of the bulb. This method dissipates unwanted IR and UV rays as heat and reflects only the desired colors, resulting in a transmitted beam of replicated daylight that can be directed toward the object of interest. The dissipated heat can often cause the original source to become quite hot, but if heat poses a significant concern, methods exist for decoupling the actual light source from beam delivery.
Choosing the right light
Physicians and dentists should have confidence that the light source they choose is appropriate and best for their needs. Color rendering index is one indicator of how good a light source is; an index of 100 is considered perfect. However, the best measure of a source’s ability to replicate daylight is its spectrum. This information is easily accessible for any commercial source, whether it’s a standard incandescent bulb or a source with a filter/coating already implemented into the design. This should be available through any reputable vendor. If the customer is sensitive enough to recognize the spectral characteristics of interest, the source’s specification data is easily interpreted to determine whether it will do the job.
In general, in a clinical or diagnostic environment, sources already implementing cold mirror coatings are probably most advantageous. They are specifically designed to dissipate UV and IR rays in the opposite direction to alleviate incidence upon the patient. The downside is that buying reflector-coated incandescent replacement bulbs, which are pre-tailored to transmit only the desirable daylight spectrum, is more costly than buying standard incandescent replacements to be implemented with dichroic filters. This enhanced performance versus added cost trade-off is inherent to all optical and illumination systems and is best left up to the doctor’s discretion.
Meet the author
Amber Czajkowski is a thin-film coating engineer at Edmund Optics in Pennsburg, Pa.; e-mail: firstname.lastname@example.org.