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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
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"ocrText": "November 18,1929.\nDr. Herman Schlundt,\nUnivirsity of Missouri,\nColumbia, No.\nDear Dr. Schlundt:\nLeeds & Northrup *phoned us on Saturday morning\nto get more information regarding the equipment which they supplied us for\nthe measuring of brightness of luminous material.\nI have referred them to our original order and their\nacceptance of same. However, I feel that it will be well to give you this\ninformation so that you may be fully aware of just what is involved.\nThe MacBeth illuminometer, as you are aware, was developed\nfor measuring light intensities in rooms 30 that it could be judged whether\nor not such sections of rooms were properly illuminated. In order to\nadopt this equipment for the measurement of the brightness of luminous mat-\nerial it is necessary to materially reduce the intensity of light measured,\nas vell as introducing the color filter to match the color of self-luminous\naine sulphide.\nThe specifications for the screens for this work are given\nin a letter of mine to leeeds & Northrup, under date of November 1,1927., copy\nof which we are enclosing.\nAt the time we ordered the equipment we also ordered some\nadditional lamps for use in our apparatus for standardization or calibration\nof the instrument. The instrument as supplied, of course, is capable of\nmeasuring comparative intensities of light only and not absolute values and,\nin order to use the instrument for absolute values it is necessary to calibrate\nsame, which involves matching with some known standard or known source of\nlight. This type of calibration the have always entrusted to the Electrical\nTesting Laboratory, Bast End Ave. and 80th St., N.Y.C.\nBy passing a definite current thru the lamp and setting the\nscale at a fixed point it is possible to ascertain what intensity of light\nis given off, expressed in terms of zicrolamberts. The intensity of the\nlight varies inversely as thesquare of the distance of the source of radiation\nand, by varying the distance of the lamp in the illuminometer it is possible\nto compute the intensity of the light as observed in the eye-piece. However,\nwhen this instrument has once been calibrated, the duration of which we may\nbe assured that such calibration is correct is very problematical and, we have\ntherefore, devised a readily portable reference standard, which may be used\nin checking the calibration of the instrument at times to make sure that\nit has not varied, and in case a variation does accur it is nuch easier to\nsend the reference standard in for recalibration or checking rather than\nthe instrument itself. We have two such reference standards and, as long\nas the reference standards agree we feel confident that our apparatus is in"
}