Calibration of a monochromator using a lambdameter T. Schwarzmaier, A. Baumgartner, P. Gege, K. Lenhard Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Methodik der Fernerkundung, 82234 Oberpfaffenhofen, Germany ABSTRACT The standard procedure for wavelength calibration of monochromators in the visible and near infrared wavelength range uses low-pressure gas discharge lamps with spectrally well-known emission lines as primary wavelength standard. The calibration of a monochromator in the wavelength range of 350 to 2500 nm usually takes some days due to the huge number of single measurements necessary. The useable emission lines are not for all purposes sufficiently dense and at the appropriate wavelengths. To get faster results for freely selectable wavelengths, a new method for monochromator characterization was tested. It is based on measurements with a lambdameter taken at equidistant angles distributed over the grating's entire angular range. This method provides a very accurate calibration and needs only about two hours of measuring time. Keywords: Monochromator, wavelength-calibration, lambdameter, wavemeter 1. INTRODUCTION The German Aerospace Center (DLR) operates the Calibration Home Base (CHB) as a facility for the calibration of airborne imaging spectrometers and field spectrometers in the wavelength range from 350 to 2500 nm [1]. For spectral measurements, an Oriel MS257 TM 1/4 m monochromator illuminated by a quartz tungsten halogen lamp (QTH) is used as light source, which has a specified wavelength accuracy of 0.15 nm. For accurate calibration results, precise knowledge of the monochromator characteristics is essential. So far, gas discharge lamps are used to determine the spectral properties. However, a full wavelength calibration takes some days due to the huge number of single measurements necessary, and is nevertheless based on few emission lines with fixed wavelengths. In order to reduce the calibration effort and to improve the coverage of reference wavelengths, an alternate calibration method was developed and tested using a spare monochromator Mc-Pherson Model 2051 with high light throughput and good wavelength accuracy. The outdated manual control was equipped with a new stepper motor with PC interface to allow computer control. Due to the complex mechanics that connects the motor with the grating, the relationship between motor position and monochromator wavelength is an unknown nonlinear function. To set a specific wavelength, a mathematical equation is needed which describes this relationship and makes it possible to calculate the motor position for each wavelength. The standard measurements with gas emission lines did not yield satisfactory results, as there were not enough useful wavelengths for an accurate polynomial fit. To get more values with sufficient accuracy, a new calibration method, which uses a lambdameter, was developed. 2. THE STANDARD CALIBRATION PROCEDURE The standard procedure for wavelength characterization uses low-pressure gas discharge lamps with spectrally well- known emission lines as primary standard (Figure 1A). Instead of the QTH lamp a gas discharge lamp (1) is installed in front of the monochromator´s entrance slit. A radiometer is mounted in front of the exit slit (8). For alignment, the grating (5) is turned into the zero order position where it works as a mirror. In this position the lamp (1) is adjusted to provide maximum radiometer signal. For calibration, the monochromator´s grating is rotated in small steps around the expected angle corresponding to the center wavelength of the selected emission line, and measurements of the relative light intensity are taken with the radiometer (Figure 2).