SAE ARP798B Design Criteria for White Incandescent Lighted Aerospace Instruments

SCOPE

This SAE Aerospace Recommended Practice (ARP) covers the general requirements and test procedures recommended for use with white incandescent integrally lighted instruments. Its use should provide uniformity of illumination from instrument to instrument and legibility under daylight operation. An appendix is provided to familiarize the designer with some of the techniques used to obtain uniformity of color and illumination in various types of instruments.

Purpose

The purpose of this document is to recommend to the designer the various criteria and to familiarize him with techniques associated with white incandescent lighted aerospace instruments.

Lighting System

The lighting system should be so designed that the light sources are contained within the instrument enclosure. The lighting system should not interfere with the visibility of any portion of the instrument display. No marking should depend solely on one incandescent lamp for its illumination. The lighting system shall contain a minimum of two lamps and shall be designed so that no portion of the display depends on one lamp for more than 60% of its illumination. The lighting system should outlast the service life of the instrument and should in no way interfere with the correct operation of the instrument.

Color

The color of the illuminated markings when 4.50 V± 0.050 V applied shall be determined on which type of “white” is called out. There are three types of “white” color selections for instrument lighting: Aviation White, Instrument and Panel Lighting White, and Blue-Filtered White. The recommended “white” for new designs should be the Blue-Filter White or commonly known as “Air Force Blue-White”. The rationale for this selection of “white” is that it can maintain a smaller color tolerance over the full brightness range. The selection of Air Force Blue-White will require filters over the lamps (in accordance to 3.2.4.3). The color coordinates for Air Force Blue-White is:

– “x” is not less than 0.420, or greater than 0.460 (using the CIE 1931 Chromaticity chart).

– “y” is not less than 0.385, or greater than 0.425 (using the CIE 1931 Chromaticity chart).

All other colors that are contained within the instrument (i.e., pointers, flags) shall adhere to color requirements of AS25050. For example, red flags should have the chromaticity coordinates of “Aviation Red”.

Brightness

In the measurement of brightness, the lamps shall be operated at 5 V±0.050 or other specified rated voltage defined by the aircraft system. Unless otherwise specified the following values of brightness in foot lamberts are recommended:

White Areas: 1.00 fL ± 0.50 fL. The overall average of all white areas shall be contained with 1.00 fL ±0.20 fL. This ensures a balanced cockpit between the other instruments.

Red Areas: 1.00 fL ± 0.50 fL.
Gray Areas: 0.60 fL ± 0.30 fL. The lower limit is due to the lower transmission levels of gray areas when compared to the white areas. This is due to the fact that one lamp may be illuminating both the white and gray areas.

Black Areas: 0.05 fL ±0.03 fL when reflected light is used (i.e., wedge and ring lighting as shown in Figures 1 and 2) and 0.04 fL ± 0.02 fL for refracted light (i.e., back lighting as shown in Figures 4 and 5).

For all other colors, the brightness limits are governed by the approval of the procuring activity. However, the general guideline is to apply the brightness limits of 1.00 fL ±0.50 fL.

For pointers, lubber lines, command bars, miniature airplane symbols and similar reference marks, a higher ilumination level should be applied to have it stand out from the other light sources within the instrument. For such markings, 1.20 fL± 0.50 fL shall be applied with the average brightness 0.10 to 0.50 fL higher than the average. This assist the operator in rading the key information components in a quick scan.

LISUN following instruments fully meet SAE ARP798B Design Criteria for White Incandescent Lighted Aerospace Instruments

LMS-6000 Portable CCD Spectroradiometer