September 1965 L E T T E R S T O T H E E D I T O R 1179

Intensity Calibration of Streak Spectrograms H A R R Y A. G I E S K E

Harry Diamond Laboratories, Washington, D. C. 20438 (Received 17 M a y 1965)

A METHOD for absolute intensity calibration of streak spectrograms has been described by Bronfin, McLean, and

Griem.1 In their method, a streak spectrograph and a mono-chromator with photomultiplier are arranged to observe simul­taneously the light from a xenon flash tube. Film calibration is accomplished through comparison of the densitometer recording of the film density with the recording of photomultiplier output at the same wavelength.

We have modified this method to eliminate the monochromator and the separate calibration exposure, and to make the intensity calibration only over the range of intensities experienced during an experiment.

The spectrograms to be calibrated are produced in a drum camera which views the focal plane of a Jarrell-Ash ƒ/6.3 ¾-m spectrograph. A beamsplitter (a thin glass plate) is placed be­tween the spectrograph and drum camera to reflect a small portion (<10%) of the light emergent from the spectrograph, through a lens, upward to a slit and photomultiplier assembly. The slit and photomultiplier are set to view the wavelength interval of particular interest in the spectral region to be recorded by the drum camera. This arrangement eliminates the need for a monochromator.

Instead of viewing a flash tube for the calibration light pulse, the luminous event itself (in this case, the light from shock-heated gas in a pressure-driven shock tube) is used as the calibration pulse. Since the film and photomultiplier view the same event, the output of the photomultiplier can be used to calibrate the film. Therefore, an intensity calibration curve for only those film densities actually obtained during the experiment is produced and the whole calibration curve is usable instead of just a small part of it. This is significant in experiments in which the light output of the luminous event is much less than that of a xenon flash tube.

Time correlation between the photomultiplier trace and densi­tometer trace is obtained by flashing a xenon flash tube in view of the spectrograph a few microseconds after the experiment has been recorded. The xenon flash produces a large fast-rise signal enabling precise time correlation.

1 R. R. Bronfin, E. A. McLean, and H. R. Griem, J. Opt. Soc. Am. 52, 224 (1962).