Apparatus and method for characterizing the luminous intensity of a lamp using a curved mirror and curved screen

Apparatus for determining the luminous intensity distribution of an automotive head light employs a curved mirror and a curved screen. A solid state camera is used to obtain a pattern of the head light output from the screen. The system requires a relatively small black box into which the head light output is directed. A beam splitter may be employed to relax the constraints on the positioning of the various components within the box. The setting of the test lamp to first and to second preset lateral angular positions at each of which a pattern is captured and the combining of the two patterns permits the use of components which are practical. The use of a linear CCD array permits economies to be obtained by rotating the test lamp or the linear array over a sequence of angular positions and by constructing a composite pattern from the patterns so generated.

FIELD OF THE INVENTION 
This invention relates to luminous intensity measurement instruments and 
more particularly to such instruments which are useful for characterizing 
the luminous intensity of automotive lamps such as head lights, tail 
lights, signal lights and the like. 
BACKGROUND OF THE INVENTION 
Automotive head lights are required to meet certain luminous intensity 
requirements established by government agencies. Accordingly, each new 
head light design has to be characterized in order to establish that it 
meets government standards. That is to say, the luminous intensity of the 
head light has to be characterized prior to production and production runs 
have to be sampled to ensure compliance. 
Standard practice in the automotive field is to employ a goniometer and to 
move the test lamp in a manner to ascertain the luminous intensity at, for 
example, each of twenty-six different angles. The system uses a single 
point photodetector which is placed at least sixty feet from the lamp 
(head light) being characterized in a totally dark room. The minimum 
distance requirement is imposed because the lamp is not a point source and 
the light from an area source at any given angle has to be measured at a 
very large distance in order to correctly ascertain the luminous intensity 
at that angular position. 
This standard practice requires a dark room of considerable size (over 
sixty feet) and takes a considerable amount of time. As a consequence, it 
is impractical to characterize each head light in a production line and 
compromises are made which result in inaccuracies in measurements and in 
aiming head lights when installed. 
New systems are being proposed to reduce the size of the dark room required 
and to speed up the time for characterizing a lamp. One such system 
employs a screen in front of a head light and uses a cooled charge coupled 
device (CCD) to take a picture of the screen. This system still requires a 
totally dark room, a distance of sixty feet, and a screen at least seventy 
feet wide although it is argued that if a lesser distance of fifteen to 
thirty feet were used, the loss in accuracy would be acceptable. A system 
of this type is described in Advances in Measurement Technology for 
Vehicle Lighting Systems by Ian Lewin, Automotive Design Advancements in 
Human Factors; Improving Driver's Comfort and Performance; International 
Congress and Exposition; Detroit, Mich., Feb. 26-29, 1996. 
BRIEF DESCRIPTION OF THE INVENTION 
In accordance with the principles of the present invention the luminous 
intensity of a lamp (illustratively a head light) is ascertained using a 
black box measuring only about six feet by four feet by four feet. The 
system employs a spherical mirror which focuses the light from the head 
light onto a spherical screen. A CCD camera (preferably but not 
necessarily cooled) is used to take a picture of the screen to determine 
the luminous intensity pattern of the head light. The output of the CCD 
camera is stored and processed to provide the desired characterization of 
the head light. The system employs a beam splitter to permit the placement 
of the screen and the camera in convenient out of the way positions. The 
system not only permits the reduction in the size of the dark room but 
also reduces the amount of time for characterization of a head light to a 
level which may permit the characterization of head lights in a production 
line. The system provides for the movement of a test head light to at 
least two different preset lateral angular positions at each of which the 
characterization is determined.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THIS INVENTION 
FIG. 1 shows a sealed box 10 in accordance with the principles of this 
invention. The box has a black interior surface and a port 11 into which 
light from a test head light is directed. The box is about six feet deep 
and six feet on a side. 
FIG. 2 shows, schematically, the components of the optical subsystem and 
the image capturing subsystem for the system of FIG. 1. Specifically, box 
10 includes a spherical mirror 12, a scatter plate, or screen 13, and a 
beam splitter 14. Beam splitter 14 is used primarily to ease the 
constraints in the placement of the mirror 12 and the screen 13 with 
respect to the test lamp. The beam splitter is preferably a 50/50 beam 
splitter and is formed on a membrane as will be discussed in more detail 
hereinafter. A solid state camera such as a charge coupled device (CCD) 
camera 15 is located to capture the light pattern formed (focused) on 
screen 13 by mirror 12. This system maps the true luminous intensity of 
the lamp at different angular directions onto the screen at corresponding 
spatial locations. 
Mirror 12 is 1.3 meters wide with a 1.0 meter radius of curvature, ideally, 
in order to achieve a measurement range of thirty degrees to either side 
of the center line of the input port in the arrangement of FIG. 1. Such a 
mirror would cost about $500,000.00 using state of the art fabrication 
technology and is therefor impractical for the systems contemplated 
herein. But, by enabling the test lamp to be preset in each of two 
different rotational positions and by capturing the luminous intensities 
at each position and by stitching together, by software, the two patterns 
so captured, a range of rotation of only fifteen degrees to either side of 
the center line of input port 11 suffices to provide the requisite 
measurement range and requires a mirror which is much smaller and of 
acceptable cost for such systems. 
FIG. 3 shows the front face 30 of box 10 of FIG. 1 along with a mount 31 
which is settable at, at least, first and second angles of fifteen degrees 
to the right and to the left with respect to the center line 33 of the 
input port as viewed in the figure. With the test lamp settable in each of 
the two positions described in connection with FIG. 2, mirror 12 need only 
be about 0.85 meters by 0.6 meters (vertical). 
Mirror 12, in the preferred embodiment, is an electroformed mirror or 
speculum and is adjustable to compensate for any unwanted movement during 
shipment. Beam splitter 14 comprises a "pellicle" which is a membrane 
stretched on a frame and having a semi-reflective film deposited on the 
membrane. Both the mirror and the pellicle are available commercially. 
Although a speculum may introduce some error in resolution due to the 
nature of electroforming, the error is within the acceptable range of one 
tenth of one degree resolution required of systems of this type. 
Screen 13 is a diffusing screen of ground glass, or translucent plastic. 
The screen preferably is spherical for systems which require high degrees 
of accuracy but could be a flat screen for less exacting lower cost 
systems. Relatively relaxed tolerances are permitted if the screen is 
curved. 
The mirror is operative to focus all light from a test lamp, which emanates 
at a given angle, on a single spot on the screen (13). A head light, in 
order to meet U.S. government standards, is tested at twenty six different 
angles, as mentioned above, A solid state camera, such as a CCD camera is 
positioned in box 10 to capture a pattern of the luminous intensity 
distribution at each of the angle settings for a test head light, as 
discussed in connection with FIG. 2, under the control of a controller 
represented in FIG. 1 as block 41. That is to say, controller 41 is 
operative to set a test head light to a first angle, turn the head light 
on, and operate the camera to acquire the pattern focused on screen 13 by 
mirror 12 for processing. It is assumed that controller 41 may comprise 
any standard computer such as a personal computer (PC) which includes 
memory and is capable of storing and processing a sixteen bit signal 
representative of a pixel of the pattern acquired. To this end, the camera 
may comprise any (preferably cooled) commercially available CCD, Charge 
Injection Device (CID) or active pixel camera. 
The operation of an illustrative CCD camera in such a system would be well 
understood by one skilled in the art and is described for head light image 
capture in the literature. Consequently, the operation of the CCD is not 
described in any further detail herein and is not necessary for an 
understanding of the present invention. The only exception is that two 
patterns are captured, in the preferred embodiment, and the camera 
subsystem needs to be able to combine the two patterns. The software to 
put the patterns together is discussed in connection with a flow diagram 
of the software hereinafter. Further, as is common of CCD systems, the 
camera has to be calibrated to compensate for variations in pixel 
sensitivity and offset characteristic of CCD cameras as is well understood 
in the art. In addition, the cameras must be calibrated against a known 
lamp standard (government accepted) in order to provide results tracable 
to a government approved standard for luminous intensity. 
The software for combining the two patterns captured by camera 15 at the 
two different lateral angular settings described in connection with FIG. 2 
is discussed in connection with the flow diagram of FIG. 5. Specifically, 
Controller 41 operates to set the test lamp to a first angular setting as 
indicated by block 51 of FIG. 5. Next, the controller activates camera 15 
to capture the pattern at that angular setting as indicated by block 52. 
The image data is calibrated and the luminous intensity pattern is 
calculated as indicated by block 53. 
Controller 41 next rotates the test lamp to a second angular setting as 
indicated by block 55, the second setting is set to provide a pattern 
which overlaps the first pattern to ensure that no gaps occur in the final 
pattern and to ensure that the patterns are easily correlated when 
combined. The pattern is then captured as indicated by block 56 and the 
pattern is again calibrated and the luminous intensity pattern is 
calculated as indicated by block 57. The overlap of the two patterns is 
then determined by correlating the luminous intensities of the two 
patterns as indicated by block 58. The luminous intensity patterns are 
then combined to form the integrated pattern based upon the overlap 
regions as indicated by block 59. If the desired angular range is covered, 
the pattern capturing procedure is completed as indicated by blocks 60 and 
61. If not, the procedure returns to block 55 as indicated by arrow 62. 
A linear CCD array could be used along with a means for rotating the array 
to sweep out an angle equal to the sweep of the two settings discussed in 
connection with FIG. 2. FIG. 4 illustrates a linear CCD array (or 
photodiode array) 45 along with a means for rotating the array as 
represented by the double-headed arrow 46. Such means for rotating 
(scanning) a linear array are well known in the literature and, 
accordingly, are not described further herein. 
The use of a linear CCD array permits the array to be rotated to capture 
the pattern of the luminous intensity distribution or, instead, to rotate 
the test lamp. In the latter case, the means for rotating described in 
connection with FIG. 3 is operative to move the test lamp incrementally 
through a sequence of angles along a rotational path under the control of 
controller 41 and the controller is also operative to synchronize the 
camera to acquire a pattern at each angle and to combine the patterns to 
form a single pattern. 
A similar software procedure is carried out for systems employing a linear 
CCD array where the test lamp is rotated to a sequence of angular 
positions and a pattern is captured at each of the positions. In this 
case, a test lamp is set in each of the angular positions discussed in 
connection with FIG. 3 and then rotated under the control of controller 41 
to consecutive angular positions for each of which a pattern is captured. 
The integrated patterns are then combined to produce an integrated pattern 
for the lamp. 
A rectangular CCD array also can be used in systems in accordance with the 
principles of this invention. Such an array would be clocked with the 
rotation of a test lamp so that the intensity pattern on any row in the 
array is synchronized with the rotation of the lamp such that a given row 
always receives the corresponding luminous intensity pattern. The array is 
operated in the familiar Time Delay Integration (TDI) mode. This provides 
an improved signal to noise ratio for the measurement and hence increases 
the measurement accuracy.