Polarization Pattern of a High Intensity Incandescent Lamp D. L Spooner

Lockheed Electronics Company, Inc., 16811 El Camino Real, Houston, Texas 77058. Received 8 May 1972.

During the evaluation of a breadboard of a continuous record­ing polarization analyzer, the intensity and polarization patterns of a small number of tungsten lamps were measured. These measurements were made primarily to check the dynamic range and settling characteristics of the instrumentation. Since de­tailed polarization data on commercially available lamps are generally not available in the literature, it was later found that these lamp polarization measurements were valuable in the design of a nonpolarized light source that was ultimately used in con­junction with the polarization analyzer in a goniopolarimeter.

The breadboard polarization analyzer employed a 25-cm focal length ƒ/11 lens to focus the lamp image on an aperture. The lamp was viewed from a distance of 84 cm. A aperture stop used in the image plane resolved an 840-µ diam portion of the lamp. This effective 840-µ measuring aperture was systemati­cally scanned on an X-Y grid by moving the bulb with an X-Y table. Light entering the scanning aperture was passed through a rotating polarizer system similar to that used by Griffith to measure Faraday rotation1 and allowed to illuminate the face plate of a 6199 photomultiplier tube. Polarization of the light was measured by determining the ratio of the filtered ac signal component (approximately 4.0 Hz) relative to the average dc signal component from the photomultiplier tube (PMT) and applying a suitable constant.

Zero polarization for system was set by using light from a port in an integrating sphere. Under such conditions a small ac signal which is due to residual polarization of the optical components and surfaces of the system is usually observed. This residual was eliminated by carefully orienting a microscope slide cover (glass so that the ac signal component is less than 0.1% of the dc signal value. One hundred percent polarization was set by placing a Polaroid HN22 polarizer over the ƒ/11 lens and setting the ac gain of the bandpass amplifier to give a 100% reading on the servorecorder which was used to perform the ratio and display functions.

The effects of current fatigue in the PMT were largely elimi­nated by a feedback arrangement similar to that of Sweet.2

By adjusting the PMT power supply voltage, the circuit main­tains nearly constant dc current at the PMT anode and, thus, very nearly constant dc voltage on the recorder slide wire. This circuit refinement made it possible to operate with three orders of magnitudes of light level variations without photometric or electrical adjustment of the analyzer. Calibration of the power supply feedback control voltage made it possible to mea­sure intensity of illumination as well as polarization level.

Generally, it was found that lamps with large enclosure volume filament volume ratios had the least polarization problem. On the other hand, lamps with small enclosure volume, filament volume ratios, such as the quartz-halogen photolamps, exhibited numerous areas of high polarization from the lamp envelope.

One of these lamps, a GE type DVY quartz bromide photo-lamp (650 W) was operated at rated voltage to give a color tem­perature of approximately 3400 K during the test. This com­bined with the spectral response of the 6199 PMT gave an over­all average spectral response centered in the yellow-green spectral region. The lamp was mounted with the primary filament helix perpendicular to the X-Y scanning plane. The lamp was scanned in the X direction approximately 25 times. Before the initiation of each X scan, the Y displacement was incremented.

2984 APPLIED OPTICS / Vol.11, No. 12 / December 1972

Fig. 1. Iso-plot of relative intensity of type DVY photo-lamp; brightest area of lamp filament taken as 100%

intensity.

Fig. 2. Iso-line plot of polarization (in percent) observed in light from type DVY photolamp.

Fig. 3. Photo of type DVY lamp in approximately the same orientation as it was when the data for Figs. 1 and 2 were

recorded.

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The strip chart recordings of these X scans were used to prepare isoline plots of polarization and intensity. Figures 1 and 2 show these ortho-iso-line plots; Fig. 3 shows a photograph of the lamp from approximately the same direction that the lamp was viewed from by the analyzer. This photo is included as an aid to interpretation of the iso-line patterns. The iso-line patterns represent a type of ortho-photo presentation; the photo of the lamp is not an ortho image and does not have exact scale and angular positional relationship to the iso-line images.

Sandus3 points out that the emission polarization increases as the emission angle increases; therefore, it is to be expected that an area of high polarization might be present at the edges of the filament. To an extent, this was found to be the case; in every case when the scanning aperture passed over the edge of a fila­ment area, a sharp polarization spike was recorded. This phe­nomenon is best illustrated by partial ring of >10% polarization around the inside edge of the filament (Fig. 2). Failure of the instrument to detect a complete ring is thought to be due to a minor misalignment of the helix and instrument axes; the maxi­mum observed value is lower than might be predicted from theory because of the scanning aperture size and the spacing between individual filament wires that make up the large filament helix.

Of particular interest is the large halo area around the filament 9% > I > 2.8% in which the observed polarization is between 10% and 40%. Simple area integration indicates that for the given viewing direction, this area represents several percent of the total flux from the lamp. Since the direction of the plane of the reflection angle varies by a full 360° over this area, it seems reasonable that the net polarization effect on the total flux output from the lamp should be small. Direct measurement of polariza­tion of the flux from the filament and this immediate region by use of a large aperture on the polarimeter shows that the average polarization is in the order of 1-2%. (The exact value seems to be highly dependent on the viewing angle.)

References 1. R. C. Griffith, Appl. Opt. 6, 772 (1967). 2. M. H. Sweet, Electron. 105 (November 1946). 3. 0. Sandus, Appl. Opt. 4, (1965).

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