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November 18,1929.
Dr. Herman Schlundt,
University of Missouri,
Columbia, Mo.
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 so 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 well as introducing the color filter to match the color of self-luminous
sane sulphide. The specifications for the screens for this work are given
in a letter of mine to Leeds & Northrup, under date of November 9,1927, 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 we have always entrusted to the Electrical
Testing Laboratory, East 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 microlamberts. 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 much 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
-2-
good working condition, but as soon as the reference standards do not agree
then we assume that there has a change come about and we at once return our
reference standards to the Electrical Testing Laboratory for recalibration.
At the time we ordered this equipment from Leeds & Northrup we had
them quote us on these reference standards, but found their price excessively
high. We, therefore, had the reference standards manufactured according to
our own specifications by the Roseville Engineering and Machine Company of
Newark, N.J. These reference standards cost approximately $25.00 each, and
should you desire such standards we would suggest that you allow us to take
care of obtaining them.
To operate the reference standards in conjunction with the controller
box of the illuminometer equipment it is necessary to have an additional
plug, the number of which, on our order of November 21,1927, is #6812.
Unless you have some means for calibrating this instrument there at the
University, I would strongly urge that you obtain reference standardsin
connection with the apparatus - otherwise, you can never be sure that your
apparatus is functioning properly. This is especially true, if you place
this instrument in the hands of your students, for it is my observation that
so long as one person who is familiar with the apparatus handles it the
apparatus remains fairly constant, but it does not take very much handling
by other people until something happens whereby you have no faith in the results
obtained.
The color gelatine film used to obtain a color match was supplied by
the Eastman Kodak Company and was ordered by us to be shipped to Leeds &
Northrup. The specification for this filter is #60-P. We ordered two (2)
square inches and the cost was approximately 800.
I trust that with this information you will feel better qualified to
proceed with the placing of your order.
In the event that I can be of any further assistance do not hesitate
to call upon me.
Sincerely yours,
Vice President.
HHBarker:
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"ocrText": "November 18,1929.\nDr. Herman Schlundt,\nUniversity of Missouri,\nColumbia, Mo.\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 so 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 well as introducing the color filter to match the color of self-luminous\nsane sulphide. The specifications for the screens for this work are given\nin a letter of mine to Leeds & Northrup, under date of November 9,1927, 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 we have always entrusted to the Electrical\nTesting Laboratory, East 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 microlamberts. 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 much 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\n-2-\ngood working condition, but as soon as the reference standards do not agree\nthen we assume that there has a change come about and we at once return our\nreference standards to the Electrical Testing Laboratory for recalibration.\nAt the time we ordered this equipment from Leeds & Northrup we had\nthem quote us on these reference standards, but found their price excessively\nhigh. We, therefore, had the reference standards manufactured according to\nour own specifications by the Roseville Engineering and Machine Company of\nNewark, N.J. These reference standards cost approximately $25.00 each, and\nshould you desire such standards we would suggest that you allow us to take\ncare of obtaining them.\nTo operate the reference standards in conjunction with the controller\nbox of the illuminometer equipment it is necessary to have an additional\nplug, the number of which, on our order of November 21,1927, is #6812.\nUnless you have some means for calibrating this instrument there at the\nUniversity, I would strongly urge that you obtain reference standardsin\nconnection with the apparatus - otherwise, you can never be sure that your\napparatus is functioning properly. This is especially true, if you place\nthis instrument in the hands of your students, for it is my observation that\nso long as one person who is familiar with the apparatus handles it the\napparatus remains fairly constant, but it does not take very much handling\nby other people until something happens whereby you have no faith in the results\nobtained.\nThe color gelatine film used to obtain a color match was supplied by\nthe Eastman Kodak Company and was ordered by us to be shipped to Leeds &\nNorthrup. The specification for this filter is #60-P. We ordered two (2)\nsquare inches and the cost was approximately 800.\nI trust that with this information you will feel better qualified to\nproceed with the placing of your order.\nIn the event that I can be of any further assistance do not hesitate\nto call upon me.\nSincerely yours,\nVice President.\nHHBarker:"
}