Documenting Shade Information for Your Laboratory

James Fondriest, DDS

(as presented to the Restorative Academy 2005)

 

Introduction

Well exposed clinical photographs document details that are missed by the eyes when shade matching. By specifically manipulating the background, exposure angle of camera to the teeth, and the camera lens depth of field (aperture) in certain ways, the practitioner can magnify the value of the information in the images. The most important parameters of a good match can be documented with three groups of choreographed images strategically composed to highlight the different dimensions of a match.

Group 1 Shape and surface morphology documentation (Figure 1)

doc-1For the evaluation of surface morphology (texture and luster), you want to maximize the surface flash reflections by using a dual point or circumferential flash and flood the image with light.

  • Black background preferred but not mandatory
  • Dry and clean surface of tooth
  • Camera lens should be oriented perpendicular to the surface being evaluated
  • Lens aperture not important in this group

 

Group 2 Translucency Documentation (Figure 2)

doc-2The amount of translucency and thickness of the enamel layer is important to document for the successful match. Translucent enamel has the property of opalescence 1-3 which can be mapped photographically. Opalescence can best be seen in lower light conditions and by using dark backgrounds 4, 5.

  • Clean teeth
  • Black background
  • Angle lens >300 from perpendicular to limit reflections.
  • Advanced photographers should set flash and camera on manual and slightly underexpose by incrementally closing down F stop which creates more visual depth and easier discernment of the layers of the target tooth. It also allows us to see into the tooth better by further lessening surface reflections.
  • For beginning photographers who prefer TTL flash, then use exposure compensation to slightly underexpose

Group 3 Chroma and Hue Documentation (Figure 3)

doc-3Chroma is the saturation of hue and is roughly inversely proportional to value. Limit reflections from the teeth and tabs and slightly underexpose the images. Use an achromatic background and crop the red gingiva to limit afterimages 6.

  • Clean tooth surface but saliva can be left on surface. Water tends to flatten surface and lessens reflections caused by surface textures.
  • Use three shade tabs in photo arranged incisal edge to incisal edge. Tabs should be parallel and equidistant to teeth from lens. Center tab is considered ideal match with other tabs one chroma stop up or down.
  • Use 18% reflective gray card background to limit glare 7.
  • Take images >300 from perpendicular to limit reflections.
  • Chroma and hue evaluation is easier with slight underexposure as described in group 2.

 

Discussion

The visual appearance of any object is determined by the quantity and quality of light coming from it. This light can be from reflection or from trans-illumination but in the dental context is primarily from reflection. Therefore competent prosthodontic replacement requires duplication of how light is reflected from the tooth. The dentist will need to document and communicate to the laboratory how the majority of this light is reflected. The characteristics that affect the quantity and quality of this reflected light, in the order of their importance are; the shape or silhouette, surface morphology, value, translucency, chroma, and lastly hue 6.

Surface reflections render the surface opaque and are the same hue as the flash 8. The amount of surface reflections in our pictures is determined by how close to perpendicular to the tooth surface you are with the flash. When documenting translucency or hue, minimize reflections by angling the camera greater than 300 from perpendicular. When we assess surface texture, it is important to maximize reflections.

Achromatic gray and black backgrounds serve to limit the distractions and color influences of the oral environment and help us see into the tooth. Gray backgrounds are restful to the cones and improve our ability to hue render without afterimages 9 and they produce less glare than black backgrounds. An extremely bright object against a dark background will interfere with our perception 5 reducing our ability to perceive visual information. This interference is generically called glare. In dental photography, the use of a black background increases impact, but it will cause glare thus masking shade mismatches. Kulzer sells Pensler Shields® in boxes of 250 (order # 50009211) which are disposable gray cardboard backgrounds.

A black background is better for translucency assessment because it allows us to see opalescence better. The blue opalescence is our cue as to the thickness and location of the translucent enamel is in the tooth. When white light goes thru the prismatic enamel, the wavelengths are bent at different angles with blue bending the most. The red-yellows transilluminate, while the blues are left within the body of enamel 1. This optical effect is called opalescence. You don’t see it well over the dentin because dentin reflects the reds and yellows back remixing with the blues to yield white light again. The black background prevents the reds from reflecting off the back of the mouth and re-adding to the blues. The black also serves to darken the blue translucent enamel (increasing contrast) against the more reflective dentin. It is important to know that there is no blue stain within the enamel and putting it in your porcelain will increase metamerism 12.

It is common with TTL photography to overexpose teeth. Because you have a dark background, the light meter increases the aperture size or slows the shutter speed letting more light in. The teeth are the brightest objects in the image and get washed out. Closing down the aperture decreases the surface brightness which allows us to see into the tooth better. The opalescent blues are more visible with slight underexposure. Underexposure and using a black background will significantly increase your ability to see the translucent enamel layer. The goal in making a porcelain restoration is to stack porcelain in layers with the same thickness and optical characteristics as the tooth. The contrived translucency photos of group 2 artificially increase the visibility of the opalescent enamel and should be used only for assessing thickness and opacity of the enamel

Summary:

By breaking down the parameters of a good match and manipulating the camera settings to increase your ability to see these parameters in your images, you can provide more information to your laboratory.

 

 

0/5 (0 Reviews)