Let’s say you just received a letter from your mother. It is in the old style — ink on paper — and delivered by the postal carrier. You hold up the letter and can easily see the characters on the page. Then you hold it behind a sheet of glass. You can still see it clearly except in a few areas. In those areas, the light from the lamp over your shoulder is refl ecting from the sheet of glass. Now let’s imagine that you had picked up a piece of frosted glass instead of a sheet of clear glass. You can now see only the larger characters on the page; and what was black is now sort of gray. Finally, let’s say you pick up a mirror. Now you cannot see the letter at all. No matter how much ink there is on the paper, all you see is the light from behind you refl ecting off the mirror.
We have just examined the concepts of front surface refl ection and a close cousin, scattering. The glass, the frosted glass, and the mirror all are refl ecting some of the incoming light back to your eyes. The clear glass refl ects the least, and the mirror refl ects the most. The frosted glass, in addi- tion to refl ecting some of the light back to you, is refl ecting bits of the light at random angles. In all cases, front surface refl ection is preventing you from having a clear view of the ink below on the letter.
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When light goes through one medium, such as air, and then encounters another medium with a different index of refraction, such as glass, plastic, or gelatin (as in photographic fi lm and paper), a portion of the incoming light will be refl ected back off the front surface. The index of refraction is the ratio of the speed of light in the given medium divided into the speed of light in a vacuum (air, in this circumstance, might as well be a vacuum). Light travels more slowly in a material with a high refractive index. The bigger the shift in refractive index, the more the light will be refl ected off the front surface. Some amount of this refl ection is unavoidable . Camera lenses suffer from front surface refl ection. Special coatings are applied to minimize the effect, but it cannot be eliminated. By the way, if the light were coming from inside the material with the high refractive index and arriving at the interface with the low refractive index, the same sort of refl ection would take place. Some of the light would bounce back into the glass instead of exiting. This is called internal refl ection; and if the angle is steep enough, the result is total internal refl ection. Fiber optics depend on total internal refl ection to keep the light inside the conductor.
Consider what you see when you watch television. There are bright areas and dark areas in the picture. Some are such that your mind calls them white and others that you consider black. But even though a television screen can get rather bright in some areas, it cannot get any darker than that same screen when the set is turned off. Yes, the gray screen of the turned-off TV is the black you think you are seeing when you watch the turned-on set. Behind the glass front of the screen there is a black shadow mask to help with colors and, to some extent, reduce glare. But what refl ects off the front surface is totally unaffected by what is on the other side of the glass. So why is the screen gray when it is not turned on? Because light is refl ecting off the front surface of the glass! It does not matter how black the shadow mask is, because light coming off the front surface of the glass never gets to the shadow mask. The refl ected light comes from light in the room, refl ects off the front of the glass, and goes into your eye. If you turn off the lights in the room, the blacks get blacker since there is very little light impinging on the front surface. And so while the percent refl ecting remains the same, the amount refl ecting is much smaller.
Front surface refl ection is unavoidable. However the degree to which it causes problems can be controlled. When images are dynamically projected or shown on a screen, the room lights can be turned down. Showing transparen- cies by projecting them also calls for a darkened room. With refl ection prints, higher room light is better, because the image has no light of its own. But it should normally be diffuse illumination — seemingly coming equally from all directions at once. Specula light (seemingly traveling in parallel beams from a certain light source) can be used if it is from only a few sources and those beams impinge at an angle of about 45 degrees to the viewing angle. In this way, the glare off the surface is reduced.
Some of the papers used to make photographic prints have a roughened surface, called a matte surface. Matte surfaces are designed to minimize the distraction caused by fi ngerprints on glossy surfaces. But the darker colors on matte-surfaced prints are not quite as dark as those on glossy-surfaced prints. Matte surfaces tend to give a “ softer ” look to the image, so they are some- times used for sentimental portraits. Consider the application of patches of paint to a test surface. We will select four different paints in two pairs. One pair is navy blue and the other is white. One of the paints in each pair dries to a glossy surface and the other to a fl at surface. The whites will appear to have virtually the same color, but the fl at blue will not be as dark as the glossy blue (unless extra pigment was added intentionally to the paint to compensate). The fl at blue appears to be slightly tinged with white (consider again the frosted glass on top of the letter from your mother).
The lesson to be learned is that front surface refl ection will limit the maxi- mum darkness we can achieve in a picture. If the display has its own light — as in a television, movie projector/screen, or slide projector/screen — we can turn off the room lights and the darks can be made darker, since there is very lit- tle stray light to refl ect off the surface. But with hard copies — such as photo- graphic prints, where turning off the room lights will make the whites go black as well as the blacks — there is little that can be done to ameliorate the dilution of the high densities. Also, fl at or matte surfaces exaggerate the effect. As a rule of thumb, it is very diffi cult to get front surface refl ections down below 0.5%. This equates to a maximum optical density of 2.3 or 1/200 of the incoming light. So even though a camera using portrait fi lm may be capable of a dynamic range of 20,000:1, the print will be limited to a dynamic range of 200:1 or less. There are several ways to get all the important information from a camera image onto a print. Some techniques include reduction of overall contrast; put- ting a curvature into the response curve; increasing the brightness in certain portions of the image or decreasing the brightness in other areas (known as dodging and burning); or, if there are several shots of the same scene at dif- ferent exposure levels, merging those images. So even if a print cannot repre- sent all the information in the original, it is important that the information is there to be enhanced and represented in the print.
To measure the optical refl ection density (log of the amount of incoming light that is absorbed — absorbance ) of a photographic image, we can use a device called a densitometer. Because of the issues raised so far, the device requires a particular geometry in order to work properly. This is called the 45–90 geometry . The device illuminates the spot to be read with a specula beam of light coming in at 45 degrees. It measures the refl ected light with a specula beam rising off the surface at 90 degrees. The bulk of the front surface refl ection will exit the spot being read at 45 degrees and only a small fraction will come off at 90 degrees. The typical front surface refl ection limit value of 0.5 degrees is read in this way. The copy stands used in laboratories to photograph items of interest are also set up to facilitate 45–90 geometry.
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The details of light going through a transmission material, such as a slide, are shown in Figure 8.1 . Similarly the details of viewing a refl ection print are shown in Figure 8.2 . Note that the transmission mode is rather simple compared to the refl ection mode. The bottom line is that in refl ection
FIGURE 8.1 Transmission Mode. Schematically the process by which light goes through a transmission sample is depicted. Some light is refl ected off the front surface and the rest goes through either a clear portion or a colored portion. The light goes through the clear potion without change and light that is not absorbed by the dye goes through the colored portion.
FIGURE 8.2 Refl ection Mode. Schematically the process by which light responds to a refl ection sample is depicted. Some light is refl ected off the front surface. The rest goes either through a clear portion or a colored portion. The light goes through the clear portion, refl ects off the white substrate, and most of it comes back out the top as white. Some of it refl ects off the layer’s top surface. The portion that goes through the color patch behaves in a similar way except that some of the light was absorbed by the dye. The front surface refl ection limits how dark the colored patch can appear.
mode there may be low density limitations and there will certainly be high density limitations.
Inks and dyes are used to make hard copy prints — either transmission (slides) or refl ection (paper-based). The overwhelming majority are refl ection prints. All refl ection prints are limited by front surface refl ections and the nature of the colorant will have some effect on this.