TECHNICAL NOTE number 263 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE 
 
 FOREST PRODUCTS LABORATORJT^ *3 ^ 
 MADISON 5. WISCONSIN ISSUED "August I960 
 
 FPL SHOW -THROUGH COMPARATOR FOR FURNITURE PANELS 
 
 ,_ US. DEPOSIT 
 
 The Forest Products Laboratory has developed all apparatus forcom- 
 paring, evaluating, and recording the surface imperfections in furniture 
 type panels having particle board cores. The apparatus is knownasthe 
 FPL Show-Through Comparator. Although the apparatus was intended 
 to be used for research involving evaluation of the show-through pattern 
 of test panels having a high -gloss black decorative laminate on each side 
 of a particle board core, it was found to be equally applicable to the 
 evaluation of furniture panels having a relatively high-gloss surface re- 
 gardless of surface color or grain. 
 
 This note describes the apparatus, sets forth the details of its manu- 
 facture, and briefly interprets the results obtained. 
 
 Basically the apparatus provides a means of observing (or photographing) 
 the reflection of a standarized grid pattern from the surface being eval- 
 uated. It consists essentially of a projector, test panel, translucent 
 screen, and a camera. Figure 1 shows in diagrammatic form a plan view 
 of the apparatus . Figure 2 is a photograph of the apparatus , with a typical 
 pattern shown on the translucent screen. For purposes of comparison 
 it is important that the area on the test surface covered by the projected 
 grid pattern be 9 inches square. The actual size of the projection slide 
 and the size of the image on the translucent screen are relatively unim- 
 portant, as they can be enlarged or reduced as desired. 
 
 Figure 3 is a replica of the master grid pattern used to produce the 2- 
 by 2-inch projection slide used by the Forest Products Laboratory. By 
 carefully photographing this figure on 35 millimeter color film, using a 
 close-up attachment to fill the slide area, additional master projection 
 slides can be easily made. 
 
 An excellent slide of the master grid was made by photographing the 
 printed page of figure 2 using color film in a 35 mm camera equipped with 
 a 50 mm lens and 3x portrait attachment. The distance from the lens to 
 the copy was about 14 inches. 
 
 Inuse this slide is projected on the test surface (as shown in figure 2) so 
 that the area covered by the projection on the test panel is 9 inches 
 square. From this surface it is reflected back to a translucent screen, 
 
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Made of thin plastic stretched tightly on a frame. (Aplastic shower- 
 bath curtain is a good source for this screen.) The focus is adjusted 
 so that the image is in sharp focus on this screen and not on the test 
 surface being evaluated. The correct focus is most easily established 
 by substituting a perfect reflective plane surface in place of the test 
 panel, such as a ground glass (used by photographers) or a ground plate 
 glass that has one surface roughened by means of sand blasting. An 
 ordinary mirror cannot be used as there will be a double reflection, one 
 from the face and one from the silvered back. There is usually sufficient 
 depth of focus so that the difference in thickness between the ground 
 glass surface used in the focusing check and the panels to be evaluated 
 is unimportant. Each grid pattern to be photographed is permanently 
 identified by a code number in the lower portion of the grid pattern printed 
 on clear glass with a felt-tipped marking pencil. 
 
 It is important that all surfaces to be evaluated, such as decorative 
 laminates and finished wood veneers be coated with a uniform film of 
 kerosene. This can be done by wiping the surface with a lint-free rag 
 which has been dipped in kerosene. The proper amount of kerosene to be 
 used canbe determined after a few trials, and should be enough to pro- 
 duce a high-gloss reflective surface but not enough to flow (and show 
 flow lines) on the vertical surface. It will take a few seconds for the 
 "wipe-marks "to disappear from the surface, after which the surface is 
 suitable for photographing for a matter of several minutes, before ex- 
 cessive drying produces a dull reflection. 
 
 Figure 4 presents photographs of four typical surfaces, which may be of 
 assistance in interpreting the grid patterns produced on the translucent 
 screen. The upper left grid is that produced by reflection of the master 
 grid from a ground plate glass surface, and represents a perfect surface. 
 The upper right grid, figure 4, is that produced by a typical particle 
 board core having a high-gloss black laminate glued to both sides, the 
 left half of the panel exposed to relatively dry conditions (30 percent 
 relative humidity)^ and the right half exposed for 2 months to relatively 
 damp conditions (80 percent relative humidity). This type of pattern is 
 typical of particle board cores , some being better and some much worse, 
 but all perhaps being best described as "mottled" or "dimpled. " 
 
 The lower left grid of figure 4 is typical of a plywood core underneath a 
 decorative laminate , again the left half after being exposed to 30 percent 
 relative humidity and the right half after exposure to 80 percent relative 
 humidity. This pattern can probably be best described as a "crossed 
 wave" pattern, the waves in each direction being caused by the grain of 
 the veneers in the core. 
 
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 Figure 3. --Master grid pattern used in FPL Show-through 
 Comparator. 
 

 
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 Figure 4. --FPL Show -through Comparator photographs of 
 four typical surfaces. Upper left is that produced by re- 
 flection of master grid from ground plate glass surface. 
 Upper right is typical particle board core. Lower left is 
 typical of plywood core underneath a decorative laminate. 
 Lower right is typical of lumber core. 
 
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The lower right hand grid of figure 4 is typical of a lumber core, again 
 the left half after being exposed to 30 percent and the right half to 80 per- 
 cent relative humidity for 2 months. Lumber cores usually produce 
 waves in one direction, parallel to the grain of the lumber core, and 
 therefore are usually described as a "wavy pattern. " 
 
 Reading the grid patterns produced in this apparatus is somewhat 
 analogous to reading X-rays, in that one must train himself to look for 
 the distortedlines and pay no attention to the general lightness or dark- 
 ness of the overall grid. A dark spot, associated with lines bent around 
 the dark area, designates a bump on the surface, whereas a light spot 
 with the lines being bent slightly towards the spot indicates a dent in the 
 surface. Along dark streak (such as that shown on the lower right hand 
 grid of figure 4) denotes a ridge on the surface, and conversely the long 
 light streak (shown on the same grid) denotes a valley in the surface. 
 Crossed light and dark streaks, such as shown on the lower left hand 
 grid of figure 4, denote crossed valleys and ridges usually typical of 
 plywood cores . 
 
 Although the grid used at the Forest Products Laboratory is shown in 
 this technical note, others who have duplicated the technique report that 
 with certain conditions a grid composed of white lines on a black back- 
 ground appears to be more effective. This may be true with certain 
 colors of test surfaces. In using the grid proposed here, a high-gloss 
 black decorative laminate is preferred over a light decorative laminate 
 as there is less "flash-back" of light to the translucent screen. Limited 
 tests have also been made at the Forest Products Laboratory with differ- 
 ent grids, one composed of twice as many lines, (the lines being half as 
 wide) and the other composed of an equal number of lines with the lines 
 being one-half the width of the standard grid shown. Both of these vari- 
 ations may have their applications, particularly on surfaces that are 
 approaching perfection. However, it has been found that the grid shown 
 appears tobemore generally satisfactory for the usual variation in sur- 
 faces produced by particle board, plywood, or lumber cores. 
 
 It is of interest to note that the technique described evaluates the sur- 
 face irregularities due to show-through of core characteristics regard- 
 less of the color or polish of the surface , within reasonable limits . This 
 suggests the modification of the equipment so that it can be used as a 
 portable apparatus to evaluate and record the surface of a piece of 
 furniture, perhaps as it leaves the factory and after certain known con- 
 ditions of use . 
 
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UNIVERSITY OF FLORIDA, 
 
 3126' 
 
 Z M 117 881