416 LETTERS TO THE EDITOR Vol. 61

Window Corrections in Spectroradiometry C. FORNO AND O. C. JONES

Division of Optical Metrology, National Physical Laboratory, Teddington, England

(Received 12 September 1970) INDEX HEADINGS: Spectroradiometry; Transmittance; Source.

In order to establish the spectral-power distribution emitted by a tungsten lamp (or by a blackbody furnace), a knowledge of the spectral transmittance of the lamp or furnace window is required.1

An approach sometimes adopted is to carry out the main experi­ment and then to remove the window from the calibrated source and measure its transmittance separately.2-5 The authors have recently studied this procedure and have concluded that it is unreliable because the window transmittance changes during the first few minutes after separation from the lamp.

Initial results were obtained from a Quinn-Barber lamp6 that had been aged for approximately 60 h at 3000 K prior to its cali­bration. Spectral transmittance was measured through 1.8-mm-diam regions of the two windows of the complete lamp; the system shown in Fig. 1 was used. The windows were then removed from the lamp and their transmittance was again measured, first separately and then in combination, spaced the same distance as in the unbroken lamp.

The product of the transmittances of the two separate windows agreed with the simulated lamp transmittances, but both results were several percent higher than the results for the complete lamp (see Fig. 2). As a check on this result, spectral transmittance was also measured on a second Quinn-Barber lamp before and after aging. The first measurement agreed with curve (c) and the second with curve (a) of Fig. 2. (The manufacturers inform us that post-manufacture checking involves less than 8 h burning.)

FIG. 1. Lamp-window transmittance-measurement system. (a) quartz iodine lamp, (b) neutral-density wedge, (c) interference filter, (d) 1.8-mm aperture, (e) Quinn-Barber lamp, (f) integrating sphere, and (g) photomultiplier.

FIG. 2. Spectral transmittances of (a) complete aged lamp, (b) oxidized windows in series, and (c) cleaned windows in series.

We hypothesized that an absorbing film of metallic tungsten, which was deposited on the windows, mainly during aging of the lamp, had oxidized after the windows were removed. This was investigated using small, gas-filled, coiled-filament lamps. These were operated at an excessive color temperature, approximately 3100 K, until a noticeable deposit appeared on the walls. Trans-mittance was measured through the envelope of each lamp and close to its center, both before and after the aging process. Measurements were limited to three wavelengths so that short-term variations could be examined.

After aging, the envelope was cracked, allowing air to enter the lamp. Measurements were made at approximately 2-min intervals for the first hour and then continued intermittently for several days after opening.

The transmittances shown in Figs. 3 and 4 demonstrate that there was a rapid increase of transmittance within 5 min of cracking the envelope, the effect being more pronounced for shorter wavelengths. The increase continued for five days, after which a slight reduction of transmittance was measured. Whether this represented a further change of the tungsten film or a small accumulation of dust inside the bulb has not been established, but the constant values for transmittance measured after nine-days' exposure indicated that the film bad become stable.

FIG. 4. Window-transmittance changes of a filament lamp at (a) 808, (b) 633, and (c) 448 nm, several days after the lamp was opened..

The change observed at a given wavelength may be described by the equation

where τt is transmittance at time t and τ0, r∞ are transmittances at t = 0 and after several days, respectively. α and k are constants that depend on the wavelength and thickness of the deposit. Attempts to relate measurements of the transmittance of the lamp (a) in the oxidized condition and (b) after cleaning with concen­trated HC1, to the transmittance of the aged lamp were unsuccess­ful, however, and no relationship was found that could be used to apply retrospective corrections to measurements made on lamp windows that had been exposed to the air. The results given in Fig. 3 may be fitted by taking α = 0.3 and k=0.7.

The transmittances of the cleaned window, aged window, and oxidized window may provide information about the physical nature of the deposit. The spectral-transmittance curves of the unoxidized tungsten film, deduced by comparing measurements on clean and blackened envelopes, show some similarity to the coloration produced by Rayleigh scattering by discrete small particles. The increase of transmittance at short wavelengths, for the film after exposure to air, suggests that an increase of the particle size may have occurred (Fig. 5). This could account for part of the observed increase of transmittance. Such a scattering effect, however, would probably cause only a small alteration of the shape of the spectral-transmittance curve and would not account for the large changes observed. A decrease of reflectance and/or absorptance of the oxide film compared to the metal film probably accounts for the over-all increase of transmittance.

Electron micrographs of thin films of vacuum-evaporated tung­sten on collodion showed that they were not continuous but con­sisted of islands from 2 to 100 nm across, the majority being 2-10

FIG. 3. Window-transmittance changes of a filament lamp at (a) 808, (b) 633, and (c) 448 nm, all shortly after the lamp was opened.

FIG. 5. Calculated spectral transmittances of window deposit: (a) tungsten film and (b) oxidized film.

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418 L E T T E R S T O T H E E D I T O R Vol. 61

nm. Partial oxidation of the film had unavoidably occurred during the 5 min between removing the specimen of film from the evaporation plant to the electron microscope but no evidence was found of a change of size of the smallest particles after nine-days' exposure to the atmosphere. The large islands, however, did show 50% increase of size. Electron-diffraction patterns taken before and after oxidation have identified the final oxide present as essentially WO3. Previous workers7 have reported in detail on the change of weight by oxidation of a thin tungsten sheet as measured with a microbalance. In particular, their result for the gain of weight of tungsten oxidizing at room temperature re­sembles our Eq. (1) for the increase of transmittance.

We conclude that erroneous estimations of lamp-window trans­mittance may result from measurements made after the window has been removed. We therefore suggest that lamps, or furnaces requiring windows, that are intended for use as standards of spectral-power distribution should be provided with similar front and rear windows on opposite sides of the filament or radiating surface. This will permit spectral-transmittance measurements to be made through both windows close to the filament region, with the lamp intact. Transmittance values for a single window could then be deduced by taking the square root of the measured values.

The authors acknowledge the help of J. Andrews in aging the Quinn-Barber lamps. This work was carried out at the National Physical Laboratory.

1 O. C. Jones, J. Phys. (D) 3, 1967 (1970). 2 K. C. Lapworth, T. J. Quinn, and L. A. Allnut, J. Phys. (E) 3, 116 (1970). 3 J. C. Flemming, Appl. Opt. 5, 195 (1966). 4 J. P. Mehkretter, Astrophysik 51, 32 (1960). 5 D. B. Judd, J. Opt. Soc. Am. 26, 409 (1936). 6 T. J. Quinn and C. R. Barber, Metrologia 3, 19 (1967). 7 E. A. Gulbransen and W. S. Wysong, A.I.M.M.E. 175, 611 (1948).