【Optical lens】How to evaluate the quality of a len

In: Lens Wiki On: Friday, April 3, 2020 Hit: 2169

When evaluating or calibrating the performance or image quality of an imaging system, an objective evaluation indicator is important. The quality of the image can be defined by different indicators, especially resolution, contrast, modulation transfer function, distortion, etc. To use these metrics, know what each of them represents and how it affects the performance of the lens.

The resolution represents the minimum size that an optical system can distinguish in a certain dimension. Taking the human eye as an example (the human eye can be considered as a camera and a zoom lens), the resolution of the eye of a healthy person without environmental impact is about 1 corner minute (1/60 degree), that is, the distance between black and white can be distinguished at a distance of one meter is about 0.29mm. So we can say that 0.29mm is the resolution of the eye at this point. The common unit of resolution is linear logarithm (lp/mm).

1/ (0.29*2) =1.7lp/mm

Line pairs are usually described in terms of their frequency: the number of lines suitable for a given unit of length. Therefore, the frequency is usually expressed as "lp/ mm" (the logarithm of black and white lines that can be distinguished within one millimeter).

2. Contrast

Contrast can be considered as the difficulty of the optical system to distinguish objects. For a certain optical system, when the object under test changes from low frequency to high frequency, the contrast always changes from high to low until it is completely indistinguishable.

IMAX represents the intensity of the bright stripe, while IMIN represents the intensity of the dark stripe. When IMAX=1 and IMIN=0, the contrast is equal to 1. When the object is photographed in detail, the boundary of the black and white stripes is no longer clear, i.e. IMAX<1, IMIN>0, so the contrast is less than 1. When the object's spatial frequency is high enough, IMAX=IMIN, the contrast is equal to zero, and the black and white stripes are completely indistinguishable.

So the question is, is contrast important or is resolution important?

First of all, what do you need the lens to do, identify, measure or detect? If it is a low precision application such as recognition and positioning, then the resolution requirements are relatively low. If it is a measurement, then resolution and contrast must be considered. Even, in some cases, the resolution can be sacrificed to find the contrast increase of a particular spatial frequency.

3, the MTF

These definitions are given, but we still need a quantifiable index that connects the two parts. This quantifiable index is usually MTF (modulation transfer function), which quantifies the extent to which the subject's regional brightness varies as it passes through the camera lens. The following example illustrates the MTF curve of the ideal lens:

A MTF of 1.0 indicates perfect contrast retention, while a value less than this indicates a loss of more and more contrast until MTF is 0 and line pairs are no longer distinguished. In the case of the same F/# and working wavelength, any optical lens has a limit curve, which only depends on the aperture of the camera lens, and has nothing to do with the number of camera pixels, which is also known as the diffraction limit.

For visual optics, the maximum spatial frequency that can be reproduced without losing more than 50 percent of MTF (" mtf-0.5 ") is an important number because it is closely related to our perception of clarity. For example, a lens with mtf-0.5 of 50 LP/mm will produce a much clearer picture than a low-quality lens with mtf-0.5 of 20 LP/mm (assuming these lenses are used for the same camera and the same aperture).

For machine vision systems, the maximum frequency corresponding to 30% contrast can be considered its resolution. When the resolution is enough, try to choose a better contrast lens, because good image quality can greatly improve the operating efficiency and accuracy of the system.

DZO dongzheng optics has a complete inspection method for materials and finished products to ensure that the differences between the products produced and the design are within the acceptable range. What we pursue is to make our lenses have higher resolution and higher contrast, so that the customer's visual system can operate more efficiently. If you have any questions about this article and our products, please contact us.

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