JPS6238647B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a brightness measuring device for an object surface, and particularly to a brightness measuring device that uses a lens system to project a beam of light from the object surface onto a light receiver.

This type of brightness measurement device is configured such that a light beam from an object plane is focused on an image plane by an objective lens, and a light receiver receives the light that passes through a perforated plate provided on this image plane. The illuminance at the image plane varies not only depending on the brightness of the object surface to be measured but also depending on the distance between the objective lens and the object surface to be measured, and there is a problem that the smaller this distance, the larger the brightness measurement error becomes.

To solve this problem, a light-shielding plate with a hole of a certain size, that is, a fixed diaphragm, is provided between the objective lens and the image plane or between the image plane and the receiver, and the receiver is fixed at a fixed solid angle. It is known that the structure is configured such that a light beam is incident on the light beam.

However, with a device equipped with such means, if you want to configure it so that the collimation distance can be shortened,
Since the diameter of the fixed diaphragm is determined by the solid angle of exit from the objective lens at the shortest collimation distance, there is a problem that the incident light flux is greatly reduced and the detection sensitivity is deteriorated. Furthermore, when measuring a minute area of a surface to be measured, a high magnification optical system or a telephoto optical system is required due to the minimum collimation distance, and the field diaphragm also requires a minute hole, which creates manufacturing difficulties. Accompany.

The present invention solves the above-mentioned problems of conventional brightness measurement devices, and its features are that the objective lens is configured by an optical system with a variable synthetic focal length, and the image plane illuminance due to the light flux passing through the objective lens is The key point is that the brightness can be measured while keeping the constant value. In the present invention, the objective lens is configured by a fixed front lens and a moving lens for focusing at the rear, or by arranging a moving lens system for focusing inside the optical system of the objective lens. It is preferable to make the focal length of the entire system variable.
If a method in which the front objective lens is extended, that is, an external focusing method is used to keep the solid angle of exit constant, the effective diameter of the objective lens becomes extremely large, and the amount by which the objective lens is extended also becomes large, resulting in poor optical performance. Become. Furthermore, due to mechanical constraints, there is a limit to the shortest sighting distance. However, when the composite focal length is changed by moving the internal optical elements as described above, such problems do not occur and favorable results can be obtained.

Hereinafter, an embodiment of the present invention will be explained with reference to the drawings. First, in FIG.
A moving lens 3 for focusing and a perforated plate 4 are arranged in this order behind the objective lens 2, and a light receiver 5 is provided to receive the light passing through the hole 4a of the plate 4. In such an optical system, if the distance between lens 2 and object 1 is S, and the combined focal length of lenses 2 and 3 is fs, the illuminance at the image plane, that is, the position of perforated plate 4, is E=K(1/fs -1/s) ² ...(1) Here, K is a constant proportional to the object surface brightness. Therefore, in order to make the image plane illuminance constant when the object plane luminance is constant, 1/f-1/s=constant=β...(2), and when the distance S is infinite, If the combined focal length of lenses 2 and 3 at this time is f _∞ , then the relationship f _s = f _∞・S/f _∞ + S ...(3) holds, so the focal length f _s can be changed to the distance S
The image plane illuminance can be made constant by changing it so as to satisfy the above-mentioned relationship in accordance with the change in . That is, when an object approaches the lens 2 as shown by the broken line 1', the lens 3 is moved forward as shown by 3' and the variation in the solid angle of exit ω is converted into the composite focal length f.
By changing _s , it is possible to keep the image plane illuminance almost constant. The above formula holds true for a theoretical optical lens with zero thickness, but in an actual lens, even if the focal length of the lens changes as described above, the image surface illuminance cannot be kept completely constant. Therefore, as the distance S changes, the image plane illuminance changes to some extent.

Next, in the two-element lens shown in FIG.
When the image of the object to be measured 1 is formed on point A by the first lens 2, the following relationship holds true.

x=f ₁ +S/f ₁ S (4) Then, when an image is formed on point B using the first lens 2 and the second lens 3, the lens distance D is expressed by the following equation.

Here, the composite focal length f _s , the magnification m and the distance s
are respectively expressed by the following equations.

f _s = f ₁ f ₂ / f ₁ + f ₂ -D ... (6) m = m ₁ × m ₂ = x / S × L-D / x - D ... (7) S = fs (1-1 /m)=f ₁ f ₂ /f ₁ +f ₂ −D(1
−s/x×x−D/LD)… (8) By substituting equation (8) into equation (3), the following equation is obtained.

Therefore, if the power is distributed so as to satisfy equation (9), the image plane illuminance can be made constant regardless of the distance S. However, in reality, it is inevitable that some distance error will occur.

FIG. 3 is a chart comparing the present invention and a conventional apparatus with respect to changes in the image plane illuminance ratio with respect to the ratio of the object distance s to the focal length f. Conventionally, as shown by curve A, the image plane illuminance ratio decreased rapidly as the distance ratio s/f decreased, but according to the present invention,
The range of small fluctuations with respect to the image plane illuminance ratio of 1.0 is extremely wide, and the range of measurable distances can be greatly increased. For example, if the allowable distance error is 4%, the conventional distance ratio s/f was limited to about 20, but in the present invention, the distance ratio s/f can be made up to about 4.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of an optical system showing an embodiment of the present invention, Fig. 2 is an explanatory diagram showing the arrangement of a two-lens configuration that can be used in the present invention, and Fig. 3 shows the effects of the present invention. This is a diagram. 1...Object to be measured, 2, 3...Lens, 4...
Perforated plate, 5... Light receiver.

Claims (1)

[Claims] 1 includes an objective lens, a perforated plate disposed on an image plane behind the objective lens, and a light receiver disposed behind the perforated plate; In the brightness measuring device in which the light that is imaged on a surface and passed through the perforated plate is received by the light receiver, the objective lens is constituted by an optical system with a variable synthetic focal length, and the light passes through the objective lens. A brightness measuring device characterized by being able to measure brightness while keeping image plane illuminance due to a light beam constant.