Monitoring camera resolution resolution

Surveillance lens resolution recognition method

Currently, megapixel HD cameras have been increasingly used in surveillance, with 1 megapixel cameras, 2 megapixel cameras, and 5 megapixel cameras. The pixel height of the camera is one aspect of the image clarity. On the other hand, a high-resolution photographic element must be equipped with a high-resolution lens to enable the HD camera's capabilities to be reflected.

Lens resolution

The resolution of the lens refers to the number of pairs of black and white lines that can be resolved within 1 mm of the imaging plane. The unit is “line pairs/mm”.

The black and white test lines of the lens pair are not infinitely distinguishable. When black and white isometric test lines do not have high density, black and white lines at the imaging plane are very clear. When black and white equal width test lines increase the density, black and white lines can be distinguished at the imaging plane, but the white lines are not so white, the black lines are not so dark, and the contrast of white lines and black lines is reduced. When black and white equal width test lines increase the density to a certain degree, the contrast of the black and white lines at the imaging plane is very small, and the black and white lines become the gray intermediate colors, which reaches the limit of the resolution of the lens.

Black and white isometric test line

The resolution of a good lens and a poor lens is very different. One way to test the lens resolution is to mount the lens under test on a film camera. To capture a black and white fringe (resolution plate), and then use a high power magnifying glass (lens resolution detector) to detect the resolution of the line counts that can be clearly resolved within millimeters of the film. The more resolution, the higher the resolution. According to our camera detection standard (JB745-65), generally, the 135 camera has a lens center field of view of 37 lines/mm and an edge field of view of 22 line pairs/mm, even if it is a first-order lens.

Determining the resolution using the method of several lines is a method, and a more objective method is to measure the MTF function of the lens.

MTF (modulationtransferfunction modulation transfer function)

In order to reflect the contrast and resolution of the lens, the difference between the brightest part of the white line and the darkest part of the black line in the measured black and white fringe pattern (resolution plate) reflects the contrast (or contrast) of the measured pattern. Let the maximum brightness of the white line be Imax and the minimum brightness of the black line be Imin. We use modulation to represent the size of the contrast. The degree of modulation M is defined as follows:

M=(Imax-Imin)/(Imax+Imin)

The difference between the brightest part of the graphic white line and the darkest part of the black line, which is also imaged by the lens, reflects the contrast (contrast) of the imaged image. Let the maximum brightness of the white line be I*max, the minimum brightness of the black line be I*min, and the modulation degree M* should be as follows:

M*=(I*max-I*min)/(I*max+I*min)

If the degree of modulation of the original image M and the degree of modulation M* after imaging after the lens, then the MTF value is:

MTF=M*/M

Of course, we hope that the modulation degree M of the original image and the modulation degree M* of the image after the shot can be “authentic” M*=M or MTF=1. At this time, the contrast (contrast) of the image after the lens is the same as the original image, but the actual situation is that the M* after the lens imaging is smaller than the original image M. M* and M are almost equal when the measured density of the black and white fringe pattern is not high. As the measured black and white fringe pattern density gradually increases, I*max and I*min will gradually approach each other, and M* will gradually decrease until it is 0, and the MTF also gradually becomes smaller until it becomes zero.

Lens MTF test results

When we change the density of the black and white fringe pattern of the test pattern, the density of the fringes of the resulting image, ie, the resolution, or line-pair per millimeter (lp/mm), also changes.

Using the lp/mm value as the abscissa, the lens MTF value is plotted as the ordinate, and we have obtained the MTF-lp/mm graph. The above is a set of gradually black and white test lines, in the middle is a set of lines through the lens imaging, the red curve in the following figure is the brightness curve of the image formed by the lens, the blue curve is the MTF-lp/mm curve. When the black and white lines are very thick (low resolution <10lp/mm), the brightness of the test chart and the imaged black and white lines are all very dark or very white. It is reflected in the red curve that the curve has a large amplitude, the blue part of the MTF. The -lp/mm curves are all close to 100%. When the test line becomes dense, the white lines imaged by the lens will become dark, and the black lines will not be too dark. See the 40lp/mm area, the red line representing the brightness of black and white will become smaller, ie the contrast will become smaller (I*max-I* Min), the degree of modulation M* becomes smaller, so the blue MTF value drops to 50%. To further improve the black and white test line density, as shown in the figure at the end of the 200lp/mm red line represents the black and white brightness of the amplitude becomes very small, tends to the black and white of the middle value, the image of the black and white line is not clear has become the middle of the gray, blue The MTF of the curve = 3%, to the resolution limit of this lens.

The human eye is aware of the contrast of 5% of fashion, and cannot detect when the contrast is less than 2%. Therefore, the lp/mm value when the MTF value is 3% is generally selected as the visual resolution of the lens.

In the figure above, there is a resolution of approximately 120 lp/mm at the intersection of the horizontal red dotted line MTF=10% and the blue line. The MTF value at a lower resolution (eg, 10 lp/mm) reflects the light transmission, contrast, and contrast of the lens. A good lens can reach more than 95%, as shown by the red curve of the MTF diagram in Figure 3 and the example in Figure 4. If the resolution is smaller at 10lp/mm, the lens represented by the green curve of the MTF diagram in Fig. 3, the MTF at 10lp/mm = 84%, its light transmittance and contrast contrast will be poor. Example, see Figure 5. Although the final resolution is similar to the lens represented by the red curve and the green curve, the transparency, contrast and contrast of the lens represented by the red curve are much better than the lens represented by the green curve.

At higher resolutions, such as 30 lp/mm, the MTF value reflects the resolution of the lens. A good lens should be more than 70%, and an ordinary lens should be less than 40%. The lens represented by the red line in Fig. 6 has an MTF of 80% at 30lp/mm, and can be seen on a high-definition camera with 1000 lines, see Fig. 7, while the lens represented by the purple line in Fig. 6 has an MTF of 30lp/mm. 48%, installed in the HD camera can only see the MTF chart above the 500 line is the curve of the MTF value at the center of the lens with the resolution lp/mm. However, MTF is also related to many factors. The aperture of the lens during the test, the distance from the test point to the center. The outer test black and white lines are parallel to the radial line (sagittal direction), and the vertical radial line is the tangential direction (meridional general direction M), and the MTF values ​​are not the same.