Optics: measuring and testing – Lens or reflective image former testing – For optical transfer function
Reexamination Certificate
2001-10-15
2004-07-06
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Lens or reflective image former testing
For optical transfer function
Reexamination Certificate
active
06760097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lens evaluation method for evaluating resolution of a lens used for a projector, where an image light including test pattern for measuring resolution is irradiated on a screen through a projection lens to display the image of the resolution-measuring test pattern on the screen and the luminance of the displayed test pattern image is detected by an image import device using an image sensor to calculate a resolution evaluation value based on the detected luminance value, and a lens-evaluating apparatus for implementing the evaluation method.
2. Description of Related Art
Conventionally, a projector including a plurality of liquid crystal panels for modulating a plurality of colors of light in accordance with image information, a cross dichroic prism for combining the color light modulated by the respective liquid crystal panels, and a projection lens for enlarging and projecting the light combined by the prism has been used.
The projection lens used in the projector may have diverse properties such as image resolution, flare and chromatic aberration on account of variation in production process thereof. Since the diversity of the properties of the projection lens exerts influence on the image displayed by the projector, the properties of image resolution, flare and chromatic aberration are evaluated before the lens manufacturer forward the lens and before the lens is assembled into a projector.
Specifically, in order to evaluate, for instance, the resolution of the projection lens, a resolution-measuring test pattern is formed on an evaluation sheet, light is irradiated on the test pattern to introduce an image light including the test pattern into the projection lens, and the image light irradiated by the projection lens is projected on the screen. The image of the test pattern displayed on the screen is detected by an image import device using an image sensor such as a CCD (Charge Coupled Device). The image detected by the device is image-processed by a computer etc. to evaluate the resolution of the projection lens.
In the above, MTF (Modulation Transfer Function) value is generally used as a resolution evaluation value for evaluating the resolution of the lens, which can be obtained according to following formula, where the maximum value of the detected luminance value of the test-pattern image is Imax and the minimum value thereof is Imin.
MTF
=(
Imax−Imin
)/(
Imax+Imin
)
However, in the above arrangement, since measured luminance used for calculating the MTF value calculated by the above formula is a relative value, the MTF value may be varied according to brightness of the image.
Further, in evaluating the resolution of the projection lens, since the image light irradiated from the projection lens to be projected on the screen tends to have the strongest intensity at the central portion to be grown weaker toward the peripheral portion thereof, even when the luminance value is obtained on a plurality of portions of the image projected on the screen, the MTF value cannot be evaluated according to the same standard.
Furthermore, when the brightness of the projected image differ according to the type of the projector, the MTF value of the respective projectors cannot be compared according to the same standard.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a lens evaluation method and lens-evaluating device capable of appropriately evaluating the resolution of the lens without being influenced by the type of projector and measured position.
In order to achieve an object of the present invention, a lens evaluation method according to an aspect of the present invention includes the steps of: irradiating an image light including a resolution-measuring test pattern onto a screen through a lens to display an image of the resolution-measuring test pattern on the screen; detecting a luminance of the image of the displayed test pattern by an image import device using an image sensor, a resolution evaluation value being calculated based on the detected luminance value; acquiring a background luminance value of a background part without the test pattern being formed by the image import device using the image sensor; acquiring a maximum luminance value in the test pattern image by the image import device using the image sensor; acquiring a minimum luminance value in the test pattern image by the image import device using the image sensor; and calculating the resolution evaluation value based on the background luminance value, the maximum luminance value and the minimum luminance value obtained through the respective steps.
A resolution chart measuring test pattern having a bright part and a dark part at a predetermined spatial frequency used in evaluating general optical system can be used as the resolution-measuring test pattern. The spatial frequency may be a plurality of spatial frequencies between 20 line/mm to 80 line/mm. Specifically, a parallel-line resolution chart may be used as the resolution-measuring test pattern, the spatial frequency being set as 20 line/mm, 25 line/mm, 30 line/mm, 50 line/mm and 80 line/mm, where two types of mutually orthogonal parallel-line resolution charts are used as one set to form the test pattern.
The above maximum luminance value refers to the luminance value at the brightest part in the test pattern image, the minimum luminance value refers to the luminance value at the darkest part, which can be obtained by conducting image processing for detecting luminance of an image such as pattern matching on the test pattern image taken by the image sensor.
The evaluation method of the present invention is suitably applied in an arrangement using an image import device with an image sensor having divergence between level-zero value on the output side and level-zero value on the input side so that level-zero light is not outputted from the output side even when the level-zero light is introduced into the input side thereby generating offset value. The image sensor may be CCD, MOS (Metal Oxide Semiconductor) sensor etc. The image import device may be an image data generator such as a video capturing board for converting an output of the image sensors into an image signal for computer.
According to the above arrangement, since the resolution evaluation value is calculated based on the background luminance value, the maximum luminance value and the minimum luminance value, even when the brightness of the image projected on the screen differs, the resolution evaluation value calculated based on the background luminance value, the maximum luminance value and the minimum luminance value taken at a plurality of positions can be evaluated according to the same standard by conducting correction processing by adding the background luminance value, so that the resolution of the lens can be appropriately evaluated without being influenced by the type of projector and location on the displayed image.
In the above, the resolution evaluation value (MTF) calculated by the evaluation value calculating step is represented according to following formula [1] where the background luminance value is represented as Io, the maximum luminance value is represented as Imax and the minimum luminance value is represented as Imin.
MTF
=(
Imax−Imin
)/(
Io*
2
−Imax−Imin
) [1]
The formula [1] can be obtained according to the following process.
When the image of the parallel-line resolution-measuring pattern C
1
as shown in
FIG. 1
is detected using an image sensor and the resolution is evaluated on the luminance value of the bright part and dark part of the pattern C
1
detected by the image sensor, the resolution evaluation value MTF can be given as a ratio between an input level as an input contrast ratio of the image light incident on the projection lens and an output level as an output contrast ratio of the image detected by the image sensor such as CCD camera, which can be calculated
Kitabayashi Masashi
Kojima Koichi
Umemura Shunji
Font Frank G.
Merlino Amanda
Oliff & Berridg,e PLC
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