ISO 12233:2024 デジタル カメラ — 解像度と空間周波数応答 | ページ 6

3.1

addressable photoelements

number of active photoelements in an image sensor (3.11)

Note 1 to entry: This equals the product of the number of active photoelement lines and the number of active photoelements per line.

3.2

aliasing

output image artefacts that occur in a sampled imaging system (3.31) due to insufficient sampling

Note 1 to entry: These artefacts usually manifest themselves as moiré patterns in repetitive image features or as jagged stair-stepping at edge transitions.

3.3

cycles per millimetre

cy/mm

spatial frequency unit defined as the number of spatial periods per millimetre

3.4

digital camera

device which incorporates an image sensor (3.11) and produces a digital signal representing a picture

Note 1 to entry: A digital camera is typically a portable, hand-held device. The digital signal is usually recorded on a removable or an internal memory.

3.5

edge spread function

ESF

normalized spatial signal distribution in the linearized (3.15) output of an imaging system resulting from imaging a theoretical infinitely sharp edge

3.6

effectively spectrally neutral

having spectral characteristics which result in a specific imaging system producing the same output as for a spectrally neutral (3.26) object

Note 1 to entry: Effectively spectrally neutral objects may have spectral reflectances or transmittances that vary with wavelength (are not constant) so long as they produce a neutral response using the specified imaging system. Objects that are effectively spectrally neutral with respect to one imaging system will not necessarily be so with respect to another imaging system.

3.7

gamma correction

signal processing operation that changes the relative signal levels

Note 1 to entry: Gamma correction is performed, in part, to correct for the nonlinear light output versus signal input characteristics of the display. The relationship between the logarithm (base 10) of the light input level and the output signal level, called the camera opto-electronic conversion function (OECF), provides the gamma correction curve shape for an image capture device.

Note 2 to entry: The gamma correction is usually an algorithm, lookup table, or circuit which operates separately on each colour component of an image.

3.8

horizontal resolution

resolution (3.23) value(s) measured in the longer image dimension, corresponding to the horizontal direction for a"landscape" image orientation, typically using a vertical or near vertical oriented test-chart feature

3.9

image aspect ratio

ratio of the image width to the image height

3.10

image compression

process that alters the way digital image data are encoded to reduce the size of an image file

3.11

image sensor

electronic device that converts incident electromagnetic radiation into an electronic signal

3.12

line pairs per millimetre

lp/mm

spatial frequency unit defined as the number of equal width black and white line pairs per millimetre

3.13

line spread function

LSF

normalized spatial signal distribution in the linearized (3.15) output of an imaging system resulting from imaging a theoretical infinitely thin line

3.14

line widths per picture height

LW/PH

spatial frequency unit for specifying the width of a feature on a test chart (3.27) relative to the height of the active area of the chart

Note 1 to entry: The value in LW/PH indicates the total number of lines of the same width which can be placed edge to edge within the height of a test target or within the vertical field of view of a camera.

Note 2 to entry: This unit is used whatever the orientation of the “feature” (e.g. line). Specifically, it applies to horizontal, vertical, and diagonal lines.

3.15

linearized

digital signal conversion performed to invert the camera opto-electronic conversion function (OECF) to focal plane exposure or scene luminance

3.16

lines per millimetre

L/mm

spatial frequency unit defined as the number of equal width black and white lines per millimetre

Note 1 to entry: One line pair per millimetre (lp/mm) is equal to 2 L/mm.

3.17

modulation

normalized amplitude of signal levels

Note 1 to entry: This is the difference between the minimum and maximum signal levels divided by the average signal level.

3.18

modulation transfer function

MTF

modulus of the optical transfer function (3.20)

Note 1 to entry: For the MTF to have significance, it is necessary that the imaging system be operating in an isoplanatic region and in its linear range. Because digital cameras (3.4) are sampled imaging systems (3.31) which use spatial colour sampling and typically include nonlinear processing, a meaningful MTF of the camera can only be approximated through the SFR. See ISO 15529 ^[4].

3.19

normalized spatial frequency

spatial frequency unit for specifying resolution characteristics of an imaging system in terms of cycles per pixel rather than in cycles/millimetre or any other unit of length

3.20

optical transfer function

OTF

two-dimensional Fourier transform of the imaging system's point spread function (3.21)

Note 1 to entry: For the OTF to have significance, it is necessary that the imaging system be operating in an isoplanatic region and in its linear range. The OTF is a complex function whose modulus has unity value at zero spatial frequency See ISO 9334 ^[2]. Because digital cameras (3.4) are sampled imaging systems (3.31) which use spatial colour sampling and typically include nonlinear processing, a meaningful OTF of the camera can only be approximated through the SFR.

3.21

point spread function

normalized spatial signal distribution in the linearized (3.15) output of an imaging system resulting from imaging a theoretical infinitely small point source

3.22

reflectance

ratio of the luminous flux reflected from the surface of the chart to the luminous flux incident on the surface of the chart. The reflectance should be integrated over the range of wavelengths from at least 400 to 700 nm.

Note 1 to entry: If the camera under test is sensitive to an extended spectral range (e.g. near Infrared wavelengths), the spectral range over which the reflectance is integrated needs to include this extended spectral range.

3.23

resolution

measure of the ability of a camera system, or a component of a camera system, to depict picture detail

Note 1 to entry: The limiting resolution, visual resolution, e-SFR and s-SFR are examples of resolution measurements.

3.24

SFR10 frequency

Spatial frequency where the SFR value drops to 10 %

3.25

spatial frequency response

SFR

relative amplitude response of an imaging system as a function of input spatial frequency

Note 1 to entry: The SFR is normally represented by a curve of the output response to an input sinusoidal spatial luminance distribution of unit amplitude, over a range of spatial frequencies. The SFR is divided by its value at the spatial frequency of 0 as normalization to yield a value of 1,0 at a spatial frequency of 0.

3.25.1

edge-based spatial frequency response

e-SFR

measured amplitude response of an imaging system to a slanted-edge input

Note 1 to entry: Measurement of e-SFR is as defined in Clause 6.

3.25.2

sinewave-based spatial frequency response

s-SFR

measured amplitude response of an imaging system to a range of sine wave inputs

Note 1 to entry: Measurement of s-SFR is as defined in Clause 7.

3.26

spectrally neutral

exhibiting reflective or transmissive characteristics which are constant over the wavelength range of interest

3.27

test chart

arrangement of test patterns (3.28) designed to test particular aspects of an imaging system

3.28

test pattern

specified arrangement of spectral reflectance or transmittance characteristics used in measuring an image quality attribute

3.28.1

bi-tonal pattern

pattern that is spectrally neutral (3.26) or effectively spectrally neutral (3.5) , and consists exclusively of two reflectance or transmittance values in a prescribed spatial arrangement

Note 1 to entry: Bi-tonal patterns are typically used to measure resolution (3.23) by using the visual resolution method.

3.28.2

hyperbolic wedge test pattern

bi-tonal pattern (3.28.1) that varies continuously and linearly with spatial frequency

Note 1 to entry: A bi-tonal hyperbolic wedge test pattern is used to measure resolution (3.23) by using the visual resolution method in this document.

3.29

vertical resolution

resolution (3.23) value measured in the shorter image dimension, corresponding to the vertical direction for a"landscape" image orientation, typically using a horizontal or near horizontal oriented test-chart feature

3.30

visual resolution

spatial frequency at which all of the individual black and white lines of a test pattern (3.28) frequency can no longer be distinguished by a human observer

Note 1 to entry: This presumes the features are reproduced on a display or print.

3.31

sampled imaging system

imaging system or device which generates an image signal by sampling an image at an array of discrete points, or along a set of discrete lines, rather than a continuum of points

Note 1 to entry: The sampling at each point is done using a finite-size sampling aperture or area.