The Science Behind How Light Affects Color When Shade Matching Teeth

The Science Behind How Light Affects Color When Shade Matching Teeth. Photo courtesy of Andrey Popov/stock.adobe.com.

The dental restorative materials available today have the most advanced esthetics possible with current technology. You know what restorative materials look like when you place them and which ones you like to use in the esthetic zone, but do you understand how light plays a role in what you see?

Chances are that you haven't thought about the science behind how light affects color since you crammed for a high school science test. However, it is essential to your restorative outcomes and what materials you choose for the job.

Light and Color

Visible light is a small part of the electromagnetic spectrum, but it is essential to perceiving color. If you passed that high school science test, you might remember that the colors we see are the light bouncing off an object back to our eyes and interpreted by our brains. An orange is orange because the fruit absorbs all the other colors of the spectrum, and only the orange wavelengths reflect.

Of course, no patient wants an orange tooth. So, how does this work with restorative materials that need to look white, or at least, not orange?

How Our Eyes Respond to Reflected Light

Light has no color. However, the light's wavelengths send messages to our brain via the retina that gives that light color. White objects reflect nearly all of the visible light. To better understand how our eyes respond to the reflected light, it is essential to understand the eye/brain interface regarding colors.¹

As light enters the eye through the pupil, it enters the retina. The retina has rods and cones that interpret light intensity (rods) and light appearance (cones). When light hits the cones, the cones send an electrical signal to the brain that interprets how that wavelength looks. However, the cones have 3 different sensitivities to wavelengths. There are red cones, blue cones, and green cones, which correspond to the type of wavelength they interpret for the brain. The red and green cones respond with signals to the brain for yellow. With blue, the blue cones respond. For the color white, all three cones respond, and the brain says, "it is white."¹

There are 3 dimensions of color our eyes see, hue, value, and chroma: ²

1. Hue is the dominant wavelength you see.
2. Value is the brightness of the light reflected.
3. Chroma is the saturation of the hue.

Light can also change appearance based on the difference between the object and what surrounds the object. For example, contrast is how different the object is from that which surrounds it. A bright object surrounded by dark colors has a lot of contrast, so the object is easier to see. However, if there is too much contrast, then it creates glare, which distorts the image and makes it harder to see.²

But What About The Way the Light Reflects?

In all cases, the color we see affects how the light bends. Since Sir Isaac Newton first discovered “ROY G BIV” from the prism, we know that light bends.

Teeth bend the light also. When clinicians are looking to restore them, they want to mimic the natural way the light reflects off the teeth and how it passes through them and transmits. When talking about fixing teeth, you must also remember the following characteristics of light and how it affects appearance:²

• Translucency is the gradient between solid and transparent objects. The more translucency you have, the lower the value you will perceive, and vice versa.
• Fluorescence describes how something absorbs light and then emits it back to the viewer. In this way, fluorescence describes one of the ways the light reflects: the more fluorescence, the lower the chroma.
• Opalescence has the effect of changing color depending upon whether light is reflecting off of it or shining through it. Opalescence bends different wavelengths in various ways, giving the object more value and enhancing the appearance of depth.

Another way light affects color involves how it reflects off the tooth's surface. Tooth surfaces have variations and curves, and how the light plays off them affects what we see, too.²

How Does The Physical Light Affect Color?

So, we know the light is color—even though it doesn't have any—because that's how our brains interpret it. However, the physical light source also affects what color we see, adding another variable to this process.

We have natural and artificial light sources today, but in the early days of humankind, there were only natural light sources, like the sun and fire. Now, we have sunlight and fire, plus a wide range of light bulbs that generate light in various ways. However, we still favor the look of color in natural light.³

The artificial light that is most like sunlight is the incandescent light bulb. Like the sun, an incandescent (and halogen) bulb heat an object to extremes to produce light. So, to our eyes, incandescent light feels the most natural to us color-wise. However, the same variable that makes them "feel right" makes them an inefficient light source.³

Fluorescent and LED lighting generates light differently. For example, LED lights produce light rays by converting electrons to photons. Therefore, the emitted light favors a specific color rather than a combination. To counteract this effect, lighting manufacturers put phosphors into them to help the light appear "white." This effort does not fool our eyes into thinking they are natural light sources. So, even though the light source seems white, LED light sources can affect what we see reflected at us off an object.³

What does the effect of the light source have on you in the dental operatory? You should use ambient light when shade matching, which is neither direct sunlight nor operatory light. In addition, you can also consider using a handheld multispectral task light to help reach ideal lighting standards for viewing color.⁴ Another idea is to shade match in the lighting that is most likely to surround the patient in their daily routine, which is particularly helpful if you do not have access to sunlight in the operatory. Plus, you can try to squint test, which involves restricting the light access by looking at the shade guide and the tooth through narrowed eyes and noting which shade fade from view first, as this is the closest match.⁵

Light and color are inextricable and essential to each other. Several years after high school, you remember why—and what you can do to ensure that the color in your patient's mouth is something that, when they see it, lights up their eyes.

Reference

1. Physics tutorial: Visible light and the eye's response. The Physics Classroom. Accessed July 8, 2022. https://www.physicsclassroom.com/class/light/Lesson-2/Visible-Light-and-the-Eye-s-Response.

2. Fondriest JF. Shade matching in restorative dentistry: The science and strategies. The International journal of periodontics & restorative dentistry. 2003;23(5)(467). doi:10.1016/j.prosdent.2004.03.015

3. Color accuracy in lighting: Artificial and natural lighting. Waveform Lighting. Published January 19, 2018. Accessed July 8, 2022. https://www.waveformlighting.com/tech/color-accuracy-in-lighting-artificial-and-natural-lighting.

4. Pitel M. September 2015. Optimizing your shade-matching success: Tips, tools, and clinical techniques.

5. Alnusayri M, Sghaireen M, Mathew M, et al. March 20, 2022. Shade Selection in Esthetic Dentistry: A Review. Cureus 14(3): e23331. doi:10.7759/cureus.23331