PHOTOCELL DISTANCE MEASUREMENT METHOD AND APPARATUS

Abstract: An apparatus and method for measuring the distance between CRT image display (11) and a photocell (12) spaced apart from the CRT, including apparatus to selectively illuminate a point (10) and a line (20) on the image screen, and apparatus which provides a distance measurement according to the ratio of the received light from the point and the line image, respectively. It is inherently independent of many sources of distance measurement error. Approximations are available which provide measurement accuracy to a predetermined level. The resulting apparatus, when mounted in an optical system, provides for the accurate adjustment of the optical system parameters and components.

Description: The present invention relates to optical distance measurement apparatus and methods, and in particular, an optical distance measurement apparatus and method applied to CRT imaging displays as is used in film recorders. CRT imaging displays measure the light output of the CRT with a photodetector. The photodetector signal is used to calibrate the brightness of the display and to adjust for aging of the CRT, temperature change and drift of operating voltages. One difficulty of this technique is that the readings from the photodetector are fairly sensitive to the distance (inversely to the square of the distance) between the detector and the phosphor of the CRT. The problem is encountered if it becomes necessary to replace the detector, the CRT, any of the mounting components, or if disassembly of the mechanical structure is required for service or maintenance. In the past, this problem has been dealt with by using very precise mechanical mounting that ensured repeatability of positions, or by mounting the photocell a considerable distance (several inches) from the CRT so that minor positioning errors are not significant. The first solution is expensive, and the second reduces the illumination on the detector and requires a more sensitive measuring device. The technique described here avoids both of these problems. The invention is an apparatus (claim 1) or a method (claim 8) which detects light intensities from any two patterns of a point a line and an area of equal luminance displayed on a surface, and calculates the distance between the detector and surface as a function of the ratio of the intensity of light from the two patterns, according to formulae which depend on the pair of patterns utilized. The present invention may be incorporated into an image recorder described in EP-A-274447. One embodiment of the invention will now be described with reference to the drawing, wherein: Fig. 1 is a geometrical illustration of the relationship of the CRT imaging display and the photodetector, including a single point illumination thereon; Fig. 2 is an illustration of a single line on the CRT image display of Fig. 1; and Fig. 3 is an illustration of the illumination of an area on the X-Y plane of the CRT of Fig. 1. A coordinate system is shown in Fig. 1. The X and Y axes are parallel to the surface of the CRT phosphor 11 which is located at Z=0. The photodetector 12 is centered above the origin at an unknown distance Z above the phosphor. Next, using Lambert's cosine law of emission, we define L as the uniform luminescence of the phosphor, ds₁ as an element of the phosphor surface, ds₂ as an element of the detector, r is the distance between the two elements, and ϑ₁ and ϑ₂ are the angles between the elements and the line connecting them (see Fig. 2). From this, the illumination of ds₂ is given by: If the photodetector is parallel to the surface of the phosphor, then ϑ₁ = ϑ₂, and this value will be called ϑ. From this equation, we can see that if we illuminate only the single point 10, Fig. 1, at the origin, then the illumination at the photodetector is proportional to: Next, if we illuminate a single line 20, Fig. 2, at Y=0, we can calculate the illumination at the photodetector as proportional to: Finally, if we illuminate the entire X-Y plane, 30 Fig. 3, we can calculate the illumination at the photodetector as proportional to: π L (4) In reality, it is impossible to illuminate a single point, line, or the entire X-Y plane. The limitation of the dot size of the CRT and the physical limits on the CRT size prevent this. However, we may make some reasonable approximations. For example, instead of illuminating a single point, assume that a small rectangular area of dimensions X₁ by Y₁ centered at the origin is used. Evaluating equation (1) shows that the intensity at the detector is approximately: and the error in this approximation is less than Similarly, instead of illuminating a line, assume that a long rectangular area of dimension X₂ by Y₂ centered at the origin is used (X₂ is much greater than Y₂). Evaluating equation (1) shows that the intensity at the detector is approximately: and the error in the approximation is less than Finally, instead of illuminating the entire X-Y plane, assume that a large rectangular area of dimension X₃ by Y₃ centered at the origin is used. Evaluating equation (1) shows that the intensity at the detector is approximately: π L = R₃ (9) and the error in the approximation is less than Thus it can be seen that if some advance knowledge is available on the expected range of values of Z and an acceptable error in the measurement is chosen, then one can easily choose sizes of the patterns to be drawn on the CRT. For example, if an accuracy of one percent is desired when Z is between Z min and Z max , then one could choose: X₂ > 10 Z max (12) X₃ and Y₃ > 14 Z max (14) It is now easy to calculate Z by means of any of the following equations: From the above, it is easily seen that if one performs any two of the three measurements, one can obtain the value of Z. This value is not in any particular physical units (e.g., inches). The conversion factor to physical units is not easily calculated since it depends on the sensitivity of the detector as a function of and on how closely the phosphor resembles a uniformly diffusing surface. This conversion factor is seldom needed, but if it is necessary, it may be determined by once taking measurements on a setup with a known value of Z. Other errors in the measurements result if the CRT phosphor does not approximate a uniformly diffusing surface, or if the sensitivity of the photodetector varies significantly with the angle of light source. If such errors are significant, then one may create a setup for measuring the ratio of the two selected patterns at various measured distances (values of Z). Then reading from an unknown system may then be looked up in this table to determine the correct value. This technique is also useful if limitations on the CRT size prevent drawing the long lengths necessary for the line and plane patterns. Moreover, this technique may also be used for measuring the thickness of the CRT faceplate on an assembled CRT. This value is important for film recorders since the glass causes the apparent position of the phosphor to be closer to the camera lens that it really is (by approximately one third the thickness of the glass). Thus the variability in faceplate thickness which require adjustments in lens focus and/or camera positioning are determined according to the process of the present invention.

Claims: A distance measuring apparatus comprising: means (11) for selectively displaying a first (10,20,30) and second (10,20,30) light pattern, wherein said first light pattern and second light pattern are a different two of a point (10), line (20), and an area (30) each having an equal luminance and constant size; a detector (12) spaced apart from said means (11) and providing two intensity signals in response to received incident light from said means (11), each of said two intensity signals corresponding to a selected light pattern; and means for calculating a measure, indicative of the distance between the said means (11) and said detector (12) according to the ratio of said intensity signals. A distance measurement apparatus according to claim 1, wherein the detector (12) is positioned along an axis normal to the means (11) which axis passes through a point at which the radiation patterns (10,20) are centred, said first light pattern approximating a point source (10) of zero dimension, said second light pattern approximating a line source (20) of one dimension, and said means for calculating provides the measure Z according to the equation: wherein X₁, Y₁ are the dimensions of the point (10), Y₂ is the smaller dimension of the line (20), and and L is the uniform luminescence of the said means (11). A distance measurement apparatus according to claim 1, wherein the detector (12) is positioned along an axis normal to the means (11) which axis passes through a point at which the light patterns (20,30) are centred, said first light pattern approximating a line source (20) of one dimension, and said second light pattern approximating an area source (30) of two dimensions, and said means for calculating provides the measure Z according to the equation: wherein Y₂ is the small dimension of the line (10), R₃ = π L, and L is the uniform luminescence of the said means (11). A distance measurement apparatus according to claim 1, wherein the detector (12) is positioned along an axis normal to the means (11) which axis passes through a point at which the light patterns (10,30) are centred, said first light pattern approximating a point source (10) of zero dimension, and said second light pattern approximating an area source (30) of two dimensions, and said means for calculating provides the measure Z according to the equation: where X₁, Y₁ are the dimensions of the point (10) R₃ = π L, and L is the luminescence of the said means (11). The distance measurement apparatus of any one of the preceding claims, wherein said light is one of infrared, visible or ultraviolet light. The distance measurement apparatus of any one of the preceding claims, wherein said means (11) comprises a CRT display system. The distance measurement apparatus of any one of the preceding claims, wherein said means for calculating includes means for compensating for angular displacement of said detector (12) and said means (11), and said means for compensating operating in accordance with the equation: wherein L is the luminescence of the means (11) displaying the light patterns, ds₁ is an element of the display surface, r is the distance between the detector (12) and the means (11) and ϑ₁ and ϑ₂ are the angles between the detector (12) and the means (11) and a line joining them. A method of measuring the distance between a light pattern on a surface (11) and a point location (12) spaced apart from the pattern, comprising the steps of: displaying a first and second light pattern at the surface, the first and second patterns being two of a point (10) a line (20) and an area (30) each having equal luminance and constant size; measuring the intensity of the light from the first and second patterns at the point location (12); and calculating a measure, indicative of the distance between the surface (11) and the location (12) according to the ratio of the measured intensity of light from the first light pattern (10,20,30) and from the second light pattern (10,20,30). A method according to claim 8 wherein said point location is on an axis normal to said surface which axis passes through the centre of the light patterns, the first light pattern approximating a point source (10) of zero dimension, and the second light pattern approximating a line source (20) of one dimension, and the step of calculating provides a measure Z according to the equation: where X₁, Y₁ are the dimensions of the first source, Y₂ is the smaller dimension of the second source, and L is the uniform luminescence of the first and second light patterns. A method according to claim 8 wherein said point location is on an axis normal to said surface which axis passes through the centre of the light patterns, the first light pattern approximating a line source (20) of one dimension and the second light pattern approximating an area source (30) of two dimensions, and the step of calculating provides a measure Z according to the equation: where Y₂ is the smaller dimension of the first source R₃ = π L, and L is the uniform luminescence of the first and second light patterns. A method according to claim 8 wherein said point location is on an axis normal to said surface which axis passes through the centre of the light patterns, the first light pattern approximating a point source (10) of zero dimension and the second light pattern approximating an area source (30) of two dimensions, and the step of calculating provides a measure Z according to the equation: where X₁, Y₁ are the dimensions of the first source, R₃ = π L, and L is the luminescence of the said means (11).

Inventors: GLASS, WILLIAM H.
Applicants: MANAGEMENT GRAPHICS, INC.
Application year: 1988
Main classification: G01S11
IPC codes: G01S11