Principles of Shade Taking and Photography
In the following excerpt from his May 2010 Inside Dentistry article, "Shade Analysis and Communication: 2010," Dr. McLaren teaches several of the essential aspects of evaluating and communicating tooth color. To read the article in its entirety, visit _____ dentalaegis.com/id.
Understanding Lighting and the Effect on Color Perception
The perception of color is affected by three primary factors: the character of the light, the observer, and the object being viewed. A change in the condition of any of the three will cause a change in the perception of color. Thus, differing viewing conditions, ie, changes in light or changes in position, can alter perception. It is impossible to try to match tooth color under every lighting and positional possibility. Thus, the clinician should try to match under the conditions that the restoration will most likely be viewed. Relative to tooth position, most people are viewed standing up at conversational distance, so this is the best position to place the patient to evaluate shade. Too often shade is taken with the patient lying back, which increases the chance of misperception. The reason this happens is the shade guides do not have the same optical properties as the natural tooth. In different viewing angles they look different; a perceived match from one viewing angle may not be a perceived match at another viewing angle. Therefore, the first rule of shade analysis is to take the shade with the patient sitting up, eye-to-eye at conversational distance.
Most shade guides were fabricated to match a standard in a 5,500 K light source. Shade guides do not have the same optical properties as natural teeth. This means they do not reflect light the same way in all lighting conditions as the corresponding shaded tooth would. Thus, visual shade matching should only be done in a lighting environment that is closest to 5,500 K. In the author’s experience, if the shade guide is matched to teeth in a 5,500 K light then it will match well in most lights, but if it was matched in a strongly biased light (eg, blue) the restoration will only match in that light.
There are many other factors that could be discussed about controlling viewing conditions. The quantity of light and the hydration of the tooth are very important. Make sure when shade matching that there are no overt shadows on the teeth or shade guide and that the light is not too strong to create specular highlights (reflective white spots). Also, the teeth need to stay hydrated. Saliva dries fast, especially with cheek retractors in. It is important to wet both the teeth and the shade guide as differences in surface texture between both can create a misperception. Using the same liquid on both surfaces can neutralize this. The second rule of shade analysis is to use full-spectrum, color-correct lighting and keep the teeth adequately hydrated.
Understanding Color Parameters Critical to Dental Shade Analysis
A basic understanding of color terminology is necessary for one to be able to evaluate differences from the shade guide but also to communicate color to the ceramist. Color has been defined in many different ways. The most widely used color-ordering or descriptive system in dentistry was developed by Mussell.1
He defined color to have three dimensions; hue, the specific wavelength of light energy that would be labeled as red, green, blue, and everything in between; chroma, the intensity, concentration, or amount of a given hue (eg, lighter yellow or deeper yellow); and value, which is the lightness or darkness of a color. In real terms, if more light reflects off an object and hits our eyes it will be perceived as brighter or higher in value; conversely, if less light reflects off an object and hits our eyes it will be perceived as darker or lower in value. There is a fourth dimension of color, translucency, that is important when evaluating tooth color because teeth are, by nature, translucent and translucency is directly related to the perception of value. When evaluating tooth color, the most important color dimension to match is the value and the translucent zones are a close second. Next in importance are the chroma zones present in the teeth being evaluated. The least important dimension of color relative to matching natural teeth is the hue. In natural teeth, the hue range is very narrow, and in the author’s experience matching the specific hue is unimportant as long as value/translucency and chroma are closely matched.
Digital Photography for Shade Communication
The second part of the author’s shade-taking technique is to record the value and chroma images using digital photography. The most important points are to use a digital SLR camera that allows interchangeable lenses; record shade images in RAW file format; and control exposure and white balance ideally with manual exposure at specific flash/subject distances.
There are four images necessary for shade communication. One image is taken with the two or three closest value shade tabs to the teeth being matched using the VITA 3D Master or the VITA Linearguide (Vident, _____ vident.com). With the VITA Classical Guide (Vident), the four closest value tabs should be in the image. Remember, the goal is to have a range of value; ideally, one tab should be slightly higher in value and one slightly lower in value. The second image is with the two closest chroma matches to the teeth. Again, one tab is slightly higher in chroma and one slightly lower. The third image is an image with what is perceived as the closest value using a small piece of digital gray card that has been attached to the shade tab. Attach the digital gray card using white utility wax. This allows for correction of inherent color bias because all flashes have subtly different color temperatures; depending on the charge state of the flash capacitor, the color temperature of the flash also can be affected. The fourth image is of the hydrated prepared tooth with a closely matched shade tab in the image. This is for the ceramist to see the preparation color to be able to modify the build-up or core color as necessary to compensate for the preparation color.
It is absolutely critical to take all of the images with the shade guide and the teeth to be matched in the same vertical plane, as objects closer to the film plane will be perceived as brighter and objects farther away will be perceived as darker. The shade guide and the teeth should be wetted with a glaze liquid. The ceramist will use this photographic information to visualize contrasts between the shade guide and the natural teeth.
Reference
1. Munsell AH. A Color Notation. 12th ed. Baltimore, Md: Munsell Color Co; 1936.
Figure 1: Image demonstrating chroma levels with the 3D Master guide.
Figure 2: Image of two of the same shade guides with different surface texture. Notice the one with different texture is perceived as a different color.
Figure 3: Using the Vita Classical Shade Guide arranged by value and working by a process of elimination to get to four tabs that cover the value range of the tooth being evaluated.
