An instrument and method for optically calibrating and balancing low level luminances of lighted instrument panel displays within the operator station of a vehicle is described which comprises a self-contained, calibrated luminance source and a beamsplitter for combining and juxtaposing an image of the calibrated luminance source with an image of the luminance from a lighted instrument panel display to be calibrated or balanced, whereby the images may be compared in luminance, the lighted instrument panel display being adjustable in intensity using the vehicle instrument panel light trim capability.

BACKGROUND OF THE INVENTION 
The present invention relates generally to systems and methods for 
calibrating or balancing low level luminance sources, and more 
particularly to an optical instrument for accurately calibrating and 
balancing low level luminances of lighted instrument panel displays. 
Calibration, adjustment and maintenance of instrument and panel lighting 
within a vehicle operator control station (such as a modem aircraft 
cockpit) is often difficult because the instrument lighting is typically 
functionally grouped and controlled by a master dimmer circuit. 
Differences among various instruments and panels make it difficult to keep 
individual instruments within a group at the same luminance level over the 
entire variable range. For example, luminances of a bank of instruments 
balanced at about 1 fL (a dusk setting) may be totally unbalanced when set 
to 0.001 fL (a dark night setting). Existing instrument panel lighting 
systems having instrument luminance trim capability have no calibrated 
standard or standardized test procedure and are adjusted manually at night 
through iterative subjective visual adjustments or with the use of a 
portable photometer, which are time consuming and expensive procedures and 
are sufficiently imprecise as to prevent assessment of the compatibility 
of a lighting system used in conjunction with an infrared-sensitive night 
vision device (NVD) (e.g., night vision goggles). 
The invention solves or substantially reduces in critical importance 
problems with existing luminance level comparator devices and methods as 
just suggested by providing a small, self-contained, calibrated, 
relatively low-cost unit that can be easily used by maintenance personnel 
to balance instrument and panel lighting equipped with light trimming 
capability. The invention may be used to evaluate the degree to which the 
lighting is compatible with NVD use. The selectable colors and luminance 
ranges permit balancing luminance levels to MIL-L-85762 specified levels 
(0.1 fL, green light for instruments and panels) and viewed with NVDs for 
possible interference (non-compatibility) with improper spectral 
characteristics. 
The invention may be used for adjustment of any standard or NVD 
instrumented, low-light level illuminated vehicle operator control station 
within cars, trucks, tanks, ships, etc, and may find particular use by 
aircraft manufacturers to quickly balance cockpit illumination, and could 
be used on a factory assembly line as an economical, quick 
check/calibration of manufactured devices having self-contained adjustable 
lighting systems (e.g., lighted instruments or panels). 
It is therefore a principal object of the invention to provide an 
instrument and method for optically calibrating and balancing low level 
luminance sources. 
It is a further object of the invention to provide an instrument and method 
for the accurate optical calibration and balancing of low level instrument 
panel luminances within a vehicle operator station. 
It is another object of the invention to provide an optical instrument and 
method for enhancing visualization of a lighted vehicle instrument and 
instrument panel display under low level luminance conditions. 
It is yet a further object of the invention to provide an instrument for 
evaluating NVD compatibility with low luminance levels of lighted 
instrument panel displays. 
These and other objects of the invention will become apparent as a detailed 
description of representative embodiments proceeds. 
SUMMARY OF THE INVENTION 
In accordance with the foregoing principles and objects of the invention, 
an instrument and method for optically calibrating and balancing low level 
luminances of lighted instrument panel displays within the operator 
station of a vehicle is described which comprises a self-contained, 
calibrated luminance source and a beamsplitter for combining and 
juxtaposing an image of the calibrated luminance source with an image of 
the luminance from a lighted instrument panel display to be calibrated or 
balanced, whereby the images may be compared in luminance, the lighted 
instrument panel display being adjustable in intensity using the vehicle 
instrument panel light trim capability.

DETAILED DESCRIPTION 
Referring now to the drawings, FIG. 1 shows a schematic block diagram of 
the essential components of a representative luminance comparator 10 
according to the invention. A principal element of comparator 10 is a 
calibrated standard light source 11. Source 11 in the representative 
embodiment shown in FIG. 1 includes a light diffusion chamber 13 including 
one or more light-emitting diodes (LEDs) 15 disposed in the walls defining 
chamber 13 at locations to substantially uniformly illuminate surface 17 
forming one end of chamber 13. It may be noted at the outset that 
alternative light sources other than LEDs may be used within comparator 
10, such as one or more incandescent or fluorescent sources or other 
source as might be selected by one skilled in the field of the invention 
practicing the invention, the specific source selection not considered 
limiting of the invention. Power source 19 is disposed adjacent chamber 13 
to provide electrical power to LEDs 15, and may for portability of 
comparator 10 comprise a battery source 20 disposed within housing 21 as 
suggested in FIG. 1. Housing 21 may enclose driver circuit electronics for 
selectively regulating luminance levels for LEDs 15 and may also include 
the necessary electronics and wiring for alternative connection to an 
external (a.c. or d.c.) power source through a power cord (not shown). A 
trigger type momentary on-off switch 22 for selectively controlling power 
source 20 may be located conveniently within housing 21 as suggested in 
FIG. 1. 
Surface 17 is preferably a flat white diffuser/reflector surface which 
reflects a beam of diffuse calibrated illumination 23, substantially 
uniform in intensity over a cross section of the beam, along optical axis 
O through a white translucent light diffuser screen 25 and removable image 
mask 27. Imaging lens 29 is disposed along optical axis O in order to 
image rumination 23 from light source 11 onto an optical combiner 
(beamsplitter) 31 also disposed along axis O. Light source 11, diffuser 
screen 25, mask 27, lens 29 and beamsplitter 31 may all be contained in 
suitable housing 32 (illustrated in FIG. 1 by peripheral broken line). 
LEDs 15 may preferably be the combination (self-contained) red-green-blue 
(RGB) type package housing three separate red, green and blue emitters. 
Incandescent, miniature fluorescent, separate LEDs or two-color LEDs (red 
and green) may also be used to produce illumination 23. LEDs 15 may be 
operatively connected to and controlled by rotary color selection switch 
33 and rotary luminance selection switch 35 so as to be individually 
selectable for red, green or blue only, plus combinations, to produce 
yellow (green+red) or white (red+green+blue) (representative LEDs 15 are 
available from Panasonic, model LN05202P, Digi-key Corp, Thief River Falls 
Minn., or Siemens, model LSPB T670, Siemens Components, Cupertino Calif.). 
Green is highly desirable for use in conjunction with use of an NVD 
compatible lighted cockpit. Yellow and red may correspond to caution and 
warning lamps, respectively. Existing aircraft may use white light 
(non-NVD compatible), or red or even blue instrument and panel lighting. 
Optical combiner 31 is disposed along axis O to optically combine and 
juxtapose images of illumination 23 projected along axis O and of 
illumination 37 projected along viewing axis V from a selected externally 
located lighted source 39 (in the form of an instrument panel display or 
the like). The spatially juxtaposed images of light source 39 and 
calibrated light source 11 may be viewed along axis V by an observer, as 
suggested in FIG. 1, at the same optical distance, which allows accurate 
comparison of the instrument (source 39) illumination 37 with the 
calibrated light source 11 illumination and the instrument illumination 
may be adjusted accordingly using any available instrument panel light 
trim capability within the vehicle. 
Image mask 27 is disposed over diffuser screen 25 and is configured to 
define an image shape corresponding to the shape of a selected instrument 
or panel light source 39 for luminance level comparative purposes as 
described more fully below. Various shaped interchangeable masks 27 may be 
used to present a reference pattern (lighted alpha-numeric display or 
other instrument display shape) corresponding to any selected source 39 
pattern, such as those shown in FIG. 3a-d, respectively, as a small 
circular light source, panel label, large light source and circular 
instrument indicia. Imaging lens 29 produces a virtual image of the masked 
image at an optical distance equal to the physical distance from source 39 
to optical combiner 31, and the image is projected into optical combiner 
31 and viewed by the observer in juxtaposition with the pattern defined by 
source 39. Table 1 lists several candidate luminance values and some 
appropriate associated color and application combinations. This optical 
image distance can be adjusted by selectively spacing lens 29 from light 
source 11. 
TABLE 1 
______________________________________ 
Standard Value 
(fL) Color* Application** 
______________________________________ 
0.001 WH, G, R I, P, H, M, N 
0.01 WH, G, R I, P, H, M, N 
0.1 WH, G, R I, P, H, M, N 
0.5 R, Y C, W 
______________________________________ 
*WH = white. G = green. R = red. Y = yellow. 
**I = instruments. P = panels. H = headup displays. M = multifunction 
displays. N = NVDC lighting. C = caution. W = warning 
In the practice of the invention in calibrating a lighted instrument panel 
display juxtaposed with calibrated light source 11, the proper color and 
pre-calibrated luminance levels (e.g., 0.5, 0.1, 0.01 or 0.001 fL) are 
manually selected (switches 33,35), and the appropriate mask 27 is 
installed in position as suggested above. Comparator 10 is held nominally 
about 20 inches from a panel light source 39 to be adjusted and lens 29 is 
placed an equivalent distance. The observer then focuses on instrument 
panel light source 39 through optical combiner 31, energizes comparator 10 
(switch 22) and compares the juxtaposed images of sources 11 and 39. The 
two images are then visually matched to the same luminance level by 
adjusting the luminance level of instrument panel light source 39 using 
the balancing circuits provided within the vehicle. 
FIGS. 2a and 2b present views as seen by the observer through optical 
combiner 31 of a transilluminated aircraft warning light, where the 
standard illuminant is the surround and the black area is the mask which 
allows the view of the lighted display, FIG. 2a being a view of the 
display where background luminance is below calibrated standard surround 
luminance, and FIG. 2b being a view of the display where background and 
standard are equal luminance. 
The invention therefore provides a small, self-contained, portable, 
low-cost, color-selectable optical instrument for accurately calibrating 
and balancing low level instrument panel luminances. It is understood that 
modifications to the invention may be made as might occur to one with 
skill in the field of the invention within the scope of the appended 
claims. All embodiments contemplated hereunder which achieve objects of 
the invention have therefore not been shown in complete detail. Other 
embodiments may be developed without departing from the spirit of the 
invention or from the scope of the appended claims.