Brightness of Grease Pencil Marks on a Vertical Plotting Board

摘要

The brightness of marked and unmarked positions on two vertical edge-lit plotting boards was measured with a Macbeth Illuminometer. One board was a Square Knock-down type at the Field Laboratory, the other was a Mark 4 Mod. 1 modified by the removal of the rear sheet of plastic. The upper seven-eights of the boards was quite evenly illuminated. Near the base, about eight inches from the lights, there was a sharp rise in brightness. Marks placed at the bottom of the board were about five or six times brighter than those near the top. On the Square Knock-down board the marks were about five times as bright as those on the modified Mark 4 board examined. China-marking pencils in white and yellow produced marks about 50-60 times as bright as the unmarked board. Other colors were relatively very dim. More light was reflected upward from a mark than in any other direction. If both sheets received the same amount of light from the lamps, marks placed on the front and rear of the board were about equally bright. The brightness of the lines etched into the plastic was about the same as a yellow grease pencil mark. A heavily marked up board contributed little to general room illumination. Over the range of 100 to 120VAC a change of one volt across the lamps produced a change in the brightness of a mark of about 0.5 apparent foot candles. (Author)