1. Field of the Invention
This invention relates to phase-type diffusing plates and, more particularly, to phase-type diffusing plates which, when used as, for example, the focusing screen for forming a finder image in the photographic camera or the like, have a predetermined diffusing characteristic that enables an excellent finder image to be observed.
2. Description of the Related Art
The conventional method of producing a certain diffusing characteristic is attained by forming a great number of minute patterns of predetermined shape in a profile of constant height on the surface of an optically transparent substrate so that the light wave emerging from the region of the minute pattern has a different phase from that of the wave emerging from the other region. Phase-type diffusing plates of the so-called bi-level form are proposed in, for example, Japanese Laid-Open patent application No. Sho 55-70827.
FIG. 10 shows a schematic fragmentary perspective view of the bi-level form of the phase-type diffusing plate in the specification of the above-identified publication. In the same figure, reference numeral 101 denotes a substrate. Minute patterns 102 of constant height are formed on the surface of the substrate 102. And, one of two parts of light which passes through a region A of the upper surface of the substrate 101 and the other which passes through regions B of the patterns 102 are given a phase difference from each other. Thus, a certain diffusing characteristic is obtained.
The light entering through the phase-type diffusing plate is diffracted to .+-.1st, .+-.2nd, . . . order maximums on either side of the 0th order or central maximum as shown in FIG. 11.
In principle, the diffracted ray "u" of a direction (.theta.x, .theta.y) of the phase-type diffusing plate is found as follows. Letting the wavelength be denoted by .lambda., and the angles of incidence by .theta.x and .theta.y, putting Nx=(sin .theta.x)/.lambda. and Ny=(sin .theta.y)/.lambda. and defining the phase distribution at a point (x, y) on the diffusing plate by .phi.(x, y), the Fourier transformation of the phase distribution gives the following equation: EQU u(Nx, Ny).about..intg..intg.exp(-i.phi.(x,y)).multidot.exp(-i2.pi.(N.sub.x .multidot.x+N.sub.y .multidot.y))dxdy
Now assuming that a plane portion A of the upper surface of the diffusing plate is taken as the reference plane, and another plane portion (also called "phase" portion) B has a height d.sub.B, and letting the refractive index of the phase portion B be denoted by n, then the phases of these plane portions are expressed by EQU .phi.(x,y).sub.A =0 EQU .phi.(x,y).sub.B =2.phi.(n-1).multidot.d.sub.B /.lambda.
Letting the area ratios of the plane portions A and B to the entirety be denoted by S.sub.A and S.sub.B respectively, the intensity I.sub.0 of the 0th order diffracted light (straight going component) at the wavelength .lambda. can be expressed in the following form: EQU I.sub.0 =1-2S.sub.A .multidot.S.sub.B .multidot.(1-(cos(2.phi.(n-1).multidot.d.sub.B /.lambda.))
where S.sub.A +S.sub.B =1
For example, quartz glass (nd=1.45867, .nu.d=67.9) is used in the substrate, and a great number of minute phase portions of height d.sub.B =0.61 .mu.m are formed on the substrate with the area ratio of the phase portions being S.sub.A =S.sub.B =0.5. With this diffusing plate, the spectral characteristic of the 0th order diffracted light becomes like that shown in FIG. 12.
In the case of the bi-level phase-type diffusing plate, it is in this condition that the intensity of the white 0th order diffracted light can decrease to the minimum. The intensity of the white 0th order diffracted light weighted with the spectral characteristic of a light source D.sub.65 and the relative luminous efficiency takes a value of 1.61% when the incident light intensity is 100%.
This example of 0th order diffracted light is tinted with blue as is understandable from the chromaticity diagram of FIG. 14. On variation of the height of the phase portions, the tint varies as depicted in the trace as shown in FIG. 14. In so low a height range d=0.5 to 0.7 .mu.m that the intensity of the white 0th order diffracted light has no practical problem,. a tint of any color from orange to blue arises unavoidably.
The presence of such a color tint in the 0th order diffracted light makes, for example, the finder image very unpleasant to observe.
In more detail, for the interchangeable lens type of a single-lens reflex camera, its finder system is designed with a condenser lens or Fresnel lens whose power is so determined that the pupils of the photographic lens and the eyepiece lens coincide with each other at the standard focal length. With such a camera, when combined with the standard lens and when the diaphragm is stopped down to small sizes of aperture opening, say F/11 or F/16 or smaller, the entire area of the finder image frame is tinted with the same color as that of the 0th diffracted light.
Further, when the standard lens is interchanged with a lens system of different exit pupil distance such as a telephoto lens or a close-up lens, and when the diaphragm is stopped down, a clear spot of the same color as that of the 0th order diffracted light appears at the center of the area of the finder image frame as shown in FIG. 15.
This phenomenon is moderated as the size of the diaphragm aperture increases. But, the telephoto lens, or close-up lens, because of being dark in F-number at full open aperture, allows it to survive in many events. Moreover, the marginal zone of the finder image frame has its diffusing light which shows the spectral characteristic of a color in complementary relation to the color of the 0th order diffracted light, which is exemplified in FIG. 13 and, therefore, is tinted with the complementary color to the color of the central zone of the finder image frame. Hence, color blurs in the finder image appear as shown in FIG. 16.
To eliminate this problem, the method disclosed in, for example, Japanese Laid-Open patent application No. Sho 55-9568 may be adopted to make the area ratio S.sub.A or S.sub.B of the phase portion deviate from 0.5 to some extent so that such a color tint just disappears. But the intensity of the 0th order diffracted light increases, thereby giving rise to an alternative problem of breaking the uniformity of the luminance over the entire area of the finder image frame.
As related art, there is U.S. patent application Ser. No. 359,487 filed on May 31, 1989.