Letters Sent to Dr. Herman Schlundt, November 18, 1929

November 18,1929. Dr. Herman Schlundt, Univirsity of Missouri, Columbia, No. Dear Dr. Schlundt: Leeds & Northrup *phoned us on Saturday morning to get more information regarding the equipment which they supplied us for the measuring of brightness of luminous material. I have referred them to our original order and their acceptance of same. However, I feel that it will be well to give you this information so that you may be fully aware of just what is involved. The MacBeth illuminometer, as you are aware, was developed for measuring light intensities in rooms 30 that it could be judged whether or not such sections of rooms were properly illuminated. In order to adopt this equipment for the measurement of the brightness of luminous mat- erial it is necessary to materially reduce the intensity of light measured, as vell as introducing the color filter to match the color of self-luminous aine sulphide. The specifications for the screens for this work are given in a letter of mine to leeeds & Northrup, under date of November 1,1927., copy of which we are enclosing. At the time we ordered the equipment we also ordered some additional lamps for use in our apparatus for standardization or calibration of the instrument. The instrument as supplied, of course, is capable of measuring comparative intensities of light only and not absolute values and, in order to use the instrument for absolute values it is necessary to calibrate same, which involves matching with some known standard or known source of light. This type of calibration the have always entrusted to the Electrical Testing Laboratory, Bast End Ave. and 80th St., N.Y.C. By passing a definite current thru the lamp and setting the scale at a fixed point it is possible to ascertain what intensity of light is given off, expressed in terms of zicrolamberts. The intensity of the light varies inversely as thesquare of the distance of the source of radiation and, by varying the distance of the lamp in the illuminometer it is possible to compute the intensity of the light as observed in the eye-piece. However, when this instrument has once been calibrated, the duration of which we may be assured that such calibration is correct is very problematical and, we have therefore, devised a readily portable reference standard, which may be used in checking the calibration of the instrument at times to make sure that it has not varied, and in case a variation does accur it is nuch easier to send the reference standard in for recalibration or checking rather than the instrument itself. We have two such reference standards and, as long as the reference standards agree we feel confident that our apparatus is in