Development of a transmissometer calibrator

Abstract

This report describes a calibrator developed for use in adjusting the transmlssometer. The results of field tests of the instrument and operating, instructions are included. With the calibrator, used carefully, the expected error in the setting of the transmlssometer is approximately P. 015 when the transmittance over the transmlssometer baseline is 0.90 or greater. 1.

Photometry of projectors at the National Bureau of Standards.

“…If Calibration, therefore, requires that, with the photocell illuminated by light from the standard lamp, the external shunt resistance is set so that the potentiometer indicates the value 5 given in equation (5). The other parameters of the calibration are usually chosen so that the shunt resistance will be of the order of a few ohms.…”

“…Equation (5) can be generalized, taking into account this condition as well as the transmittance of any filters or sector disks used in calibrating, so that where y is the transmittance of the color filter at the color temperature of the standard lamp, y is the transmittance of the sector disk, and Fî s the ratio of the output of the light under test when it is operated under the design conditions to the output of the light when it is operated under test conditions. This ratio may be, for example, the ratio of the rated lumen output of the test lamp to the output of the lamp at the test voltage.…”

Visual range :

“…As the RVR minimum was reduced, better information of visibility conditions along the runway beyond the touchdown zone became necessary, and transmissometers were installed at the midpoint of some runways [113a] . Calibrators designed to replace the visual estimates used in adjusting the full scale (perfectly clear weather) reading of the transmissometer were developed and are now coming into service [29,75] . However, the basic transmissometer, the contrast and illuminance thresholds, the illuminance level for transition from day to night scales, and the reporting increments have not been changed since the first use of the RVR system, nearly 20 years ago.…”