A FLUORESCENT ULTRAVIOLET PHOTOMETER

BY WI. T. ANDERSON, Jr., AND ELIAS GORDON

Recent years have seen a widespread employment of ultravioletlight in industry and medicine. With this increase has arisen a con-stantly increasing demand for a simple, practical and reasonablyaccurate method for measuring this form of energy. The laboratorymethods for measuring ultraviolet are not at all practical for, thephysician who is desirous of knowing the intensity of his artificiallight source in order to calculate dosage; nor is this equipment practicalfor the average industrial organization which employs ultraviolet energy.Very many different methods for measuring ultraviolet are at presentin use, practically all of which depend on some form of chemicalchange. Thus, many substances decompose in ultraviolet radiation,the amount of decomposition being proportional to the intensity of theradiation. Many of these methods, if carefully performed, are veryaccurate. However, they require a certain amount of chemical skilland time for their operation, and consequently are suitable for only afew. The matching of faded dyes and darkened photographic paper tostandard samples is another favorite method. These tests, however,are neither accurate nor dependable.

We have been studying, since 1924, methods available for measuringultraviolet. While engaged in these studies it was found that fluorescenceof certain substances excited by ultraviolet radiations can be matchedby a variable comparison light source. This was in effect a fluorescentphotometer which, after calibration, could serve as a simple andreasonably accurate means for measuring the intensity of ultravioletlight from a light source.

The light emitted by fluorescent substances is of such low intensitywhen compared to the light from lamps or natural daylight, that it isnecessary to observe it in darkness or in dimmed illumination.

Since all practical methods for producing ultraviolet radiations alsoproduce visible light it is necessary to enclose the fluorescent unit ofthe photometer in a light-tight container provided with a filter whichtransmits the exciting ultraviolet, but not the visible light from thelight source. The best available filter is purple Corex, G986A. Theultraviolet transmission of this filter is shown in curve B of Fig. 1. In

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ULTRAVIOLET PHOTOMETER

addition to the ultraviolet it transmits a little of the violet, red andinfrared. It is absolutely opaque to yellow and green light.

The most suitable fluorescent material for a practical photometerappears to be uranium glass; the curve A of Fig. 1 indicates the intensityof the fluorescent emission with exciting wave length. It is especiallywell adapted for use with the purple Corex for it responds to all theultraviolet wave lengths transmitted by the Corex and emits in theyellow-green to which portion of the spectrum the Corex is completelyopaque. This latter feature enables accurate observation of the fluo-rescent light.

A'

~~~~A0 ~ ~ ~ 30

Wave length - angstrom Wits

FIG. 1.Curve A-Responsivity of uranium glass to exciting radiation in terms of maximum.Curve B-Ultraviolettransmission of Corning G986A glass.

Variations in the transmission of the ultraviolet filter and in theresponsivity of the uranium glass causes the intensity of the observedfluorescent light to be dependent not only on the intensity of the ultra-violet source but also upon the spectral distribution of the ultravioletenergy in the source. This latter necessitates that a photometer com-prising these units, Corex filter and uranium glass, be calibrated againsteach type of light source with which it is to be employed.

Before entering into a discussion of calibration it is advisable toconsider a practical form of fluorescent photometer such as is shown inFig. 2. The photometer consists of a light-tight box. A is a movabledisk in which there are cut a number of slit openings of uniform lengthand varying widths. By bringing the proper opening into position overthe ultraviolet filter B it is possible to control the area of light incident

Mar., 1928] 225

W. T. ANDERSON, JR., AND E. GORDON [J.O.S.A. & R.S.I., 16

on the fluorescent uranium plate C. This area of light, as well as the in-tensity of the light, determines the brightness of the fluorescent lightobserved by the operator. These various slit openings enable thephotometer to be used with a very much greater range of intensitiesthan would otherwise be possible. Adjacent to the fluorescent plate andseparated from it by an opaque partition E is a comparison plate ofground white glass D which is illuminated by a 110 volt incandescentbulb G, the intensity of which is controlled by a variable resistance (notshown in illustration) mounted on the meter box. The current con-sumption of the comparison lamp is indicated by a milliammeter (notshown in illustration) mounted on the side of the photometer. Thefluorescent light and the comparison light are reflected by the mirrorF through the monochromatic green filter H, and into the eye-piece K.

FIG. 2. Special arrangeoment for fluorescent photometer.

The calibration of this photometer is a very complicated procedure.It is necessary to measure the total ultraviolet from the light sourcefalling on the slit of the photometer. This in itself is a problem for thereis no filter available which transmits only the ultraviolet and 100 percent of the ultraviolet. It was found possible to measure the totalultraviolet by a roundabout procedure which consisted of a determina-tion of the spectral energy distribution of the ultraviolet of each lightsource, the determination of the relative intensity of the spectralizedlight through Corning G986A (purple Corex) glass, and a quartz watercell to absorb long infrared energy, and the measurement of the totallight from the source through this filter and water cell and throughthe filter, water cell and Corning G24 red glass. Employing the dataobtained from these measurements it is possible to calculate with con-siderable accuracy the total ultraviolet falling upon the slit of the

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ULTRAVIOLET PHOTOMETER

photometer. The procedure in detail was as follows:-the light fromthe source with which the meter was being calibrated was passed intoa large corrected quartz prism monochromator, spectralized, and theradiations from 4100 to 2450 A.U. divided into 14 blocks of wavelengths. A portion of the visible violet is included because the ultra-violet filter transmits these wave lengths to a slight extent. Each oneof these blocks of lght was reduced to a definite area and projected on aBi-Ag thermopile and galvanometer deflections noted. The sum of thegalvanometer deflections for all the blocks represents the total intensityof the ultraviolet transmitted by the monochromator. The transmis-sion of the G986A glass and the quartz water cell for each of these blockswas determined by passing the light of each block through it and theninto the pile, maintaining the same area on the pile. The sum of thedeflections for all these blocks of wave lengths represents the totalultraviolet transmitted by the filters. The relative amount of lighttransmitted by the filters is represented by the ratio of the deflectionsfor the filtered light to the deflections for the unfiltered light.

If it were not for the fact that the G986A filter and quartz water cellin addition to the ultraviolet transmits a little red and infrared, thetotal ultraviolet intensity from the light source would now be determinedby passing the light through the filter and water cell and measuringthe transmitted energy with a Bi-Ag thermopile and galvanometerwhich have been calibrated against a standard radiation source, thefollowing proportionality holding:

Total intensity of 14 blocks Total ultraviolet intensity of lampTotal intensity of 14 blocks through filter - Measured intensity of lamp through filters

It is necessary before the above proportionality can be employed tocorrect the "measured intensity of lamp through filters" for red andinfrared light. This can be accomplished by interposing in addition tothe ultraviolet filter and water cell a plate of red G24 Corning glass.This glass transmits only the red and infrared, and can be calibratedagainst the light source with which it is to be used.

Table 1 illustrates the determination of the total ultraviolet from aquartz mercury vapor arc lamp. It can be seen that when the pre-liminary measurements have been made the total ultraviolet intensityof the light source at any point may be rapidly determined by measuringthe light passed through the G986A filter and the quartz water cell,and also through the filter, water cell and G24 glass. This latter valuemultiplied by the factor shown in the table corrects the component in-

Mar., 192 8] 227

228 W. T. ANDERSON, JR., AND E. GORDON [J.O.S.A. & R.S.I., 16

tensity which may then be subtracted from the value of that throughG986A filter and the water cell alone. The figure obtained in this way,and the other proper figures, are substituted in Equation 1 and theequation solved for total ultraviolet intensity of the light at the point

TABLE 1. Total Ultraviolet front quartz mercury arc at a distance of 40".(Old lamp, operating at 75 volts, 3.5 amps.)

Block ofwavelengths-

Angstromunits

4100-40004000-38003800-36003600-32003200-31003100-30003000-29502950-29002900-27802780-27202720-26802680-26002600-25202520-2450

Distributionof energy inmm deflec.

1618

44244

2821365730511827949736

Percentagedistribution

10.80.5

29.82.9

19.09.13.82.03.41.21.86.36.52.4

Distributionthrough filters

114

26328

1727630131966

1131

Percentagetransmission

through filter,G986A, & water

cell

75060646156534337332212

.33

Ergs/per sec/per mm2

at 40"

2.840.137.830.764.992.391.000.530.900.310.471.651.710.63

4100-2450 1483* I 100. 643 44.5 - 26.29

* Totals are what they should be, not the actual addition values.

Total Intensity of 14 blocks ="4 it CC 14 " through filters =

Measured Intensity of lamp through filters ="C it CZ " through filters and G24 =

Infrared factor= 1.32; 1.32X2= (approximately94-3 = 91, ultraviolet energy

1483 mm defl.643 " a

94 {C "

23 ,s

1483 X643=91; X=212 mm defl., total ultraviolet energy at 40"643 91'

By Calibration against standard lamp, 1 mm defl. is equivalent to 0.124 ergs/per sec./per mm'.212X0.124=26.29 ergs/per sec./per mm2

at which the measurements were made. If then the photometer slit isplaced in the position formerly occupied by the thermopile and thefilterand water cell are removed, other conditions remaining unchanged,a slit opening and reading of the milliammeter will be found which cor-

ULTRAVIOLET PHOTOMETER

responds to the particular light intensity. These measurements must.be repeated through a whole range of intensities so that every slit inthe disk is employed a number of times, with a number of readings onthe milliammeter. The instrument is then calibrated against the par-ticular light source and the calibration may be such that the milliam-meter readings for a particular slit are immediately converted intoabsolute units, ergs/per sec./per mm2 by reference to a table. Whenonce calibrated in this way the meter may be employed for measuringthe intensity of the ultraviolet light from all sources of this type. Inorder to be employed with other types of sources it must be calibratedagainst them in the manner described above.

If every photometer had to be calibrated in this manner it would beentirely impractical to make any quantity of these instruments.

FIG. 3. Fluorescent photometer in practice

Fortunately, however, it is possible to calibrate a standard instrumentand refer other instruments to this standard.

It is obvious that the method of calibration as above adoptedeliminates any variability introduced because of the unequal trans-mission of the ultraviolet filter and the variable responsivity of theuranium glass. In a similar way variations in the composition of thevarious batches of the glasses will in no way affect the accuracy of themeters.

The photometer is of special value to the physician who wishes tomeasure the intensity of his ultraviolet lamp each time a treatment is

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