Optics-related knowledge in camera modules (1)

September 8, 2023

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First, let’s briefly understand the concept of camera module we are going to talk about. Camera module, full name Camera Compact Module, abbreviated as CCM, is an important electronic device for image capture.

1. TCP (correlated color temperature)

Correlated color temperature is the temperature of a complete radiator (blackbody) when the chromaticity of a certain light source (gas discharge light source) is closest to the chromaticity of a complete radiator (blackbody) at a certain temperature, referred to as correlated color temperature. The symbol is Tcp, and the unit is Kelvin (K).

2. TCP (color temperature)

Color temperature is a unit of measurement that represents the color components contained in light. Theoretically, the blackbody temperature refers to the color of an absolute blackbody when it is heated from absolute zero (-273°C). After being heated, a black body gradually changes from black to red, to yellow, to white, and finally emits blue light. When heated to a certain temperature, the spectral component of the light emitted by the black body is called the color temperature at this temperature, and the measurement unit is "K" (Kelvin).
If the light emitted by a certain light source has the same spectral component as the light emitted by a black body at a certain temperature, it is called a certain K color temperature. For example, the color of the light emitted by a 100W light bulb is the same as the color of an absolute blackbody at 2527°C. Then the color temperature of the light emitted by this light bulb is: (2527+273) K = 2800K.

In fact, there is no difference between the two meanings of TCP above. They are generally called color temperature. Don’t get too confused.

3. MTF (modulation transfer function)

Modulation Transfer Function is a more scientific method for analyzing the resolution of a lens. Recently, more and more people have discovered that although this is a standard, some images cannot be measured by standardization, so MTF is just a reference value. Not all.
This method of measuring optical frequency is measured by how many lines can appear within a range of mm, and its unit is expressed in line/mm. So when a lens can achieve the point where what goes in is what comes out, it means that the lens is a so-called perfect lens. However, because the design of the lens often has many factors, it is impossible to have such an ideal lens.

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MTF cut off (MTF cut off frequency): OQAS parameter, indicating that the MTF curve of the human eye will reach the resolution limit when the spatial frequency reaches this frequency value, that is, the MTF value tends to zero.
Normal person ≧30c/deg, the larger the value, the better the visual quality.


4. SFR (Spatial Frequency Response)
There are many units (5) of spatial frequency, among which cycles/pixel (cycle per pixel) is the imaging system in digital cameras.
SFR is the abbreviation of spatial frequency response (Spatial frequency response), which refers to the amplitude response of a system relative to the input spatial frequency. For camera systems, SFR is similar to the MTF (modulation transfer function) of traditional optical systems. ), the resolution capability of the system can be determined intuitively.
The relevant definitions and test methods of SFR were formulated and standardized by the International Standardization Organization (ISO).
What is spatial frequency?
The general frequency in our daily life refers to the time frequency, its unit is Hz, and its definition is the number of movements per unit time (s).
[Official definition: 1. The number of times or weeks of vibration or oscillation completed per unit time; 2. The number of times something occurs or the number of times a process is completed within a certain period of time]
The dividend here is unit so it is the time frequency.
But what if we change the unit of the dividend to the spatial unit millimeter? It is the number of times something appears in the unit millisecond. Here we call it the spatial frequency.
[Official statement: But the expression of spatial frequency is customary to use "line pairs per millimeter". (LP/mm), that is, the number of line pairs per millimeter of width. ]
[It can also be expressed by these: The unit of spatial frequency is generally

Line pairs per millimeter (lp/mm),
Cycles per millimeter (cycles/mm),
Cycles per pixel (cycles/pixel),
Line Widths per Picture Height (LW/PH Line Widths per Picture Height),
Line pair per image height (lp/ph). ]
Among them, lp/mm is the most commonly used unit at present. cycles/pixel is the imaging system in digital cameras. The next pixel of a digital camera is 1 cycles/pixel, two pixels is 0.5 cycles/pixel, and four pixels is 0.25 cycles/pixel.
So what are MTF and SFR, or what are the functions of MTF and SFR? Here we have to talk about the resolution of the camera.
The resolution of the camera is to analyze the effect of the camera taking images of different spatial frequencies. The clearer the image of different cameras at high spatial frequencies, the higher the resolution.
MTF (modulation transfer function):
How to calculate the modulation transfer function of a camera: (It is to obtain a function based on analyzing the imaging effect (MTF value) of the same camera at different spatial frequencies)
The calculation of MTF is to calculate the contrast between the brightest and darkest line pairs. (Calculation process: the camera images images at spatial frequency A, and reads its brightest and darkest values)
The calculation formula is:
MTF = (maximum brightness - minimum brightness) / (maximum brightness + minimum brightness)
The obtained MTF value is the resolving power of the camera at the spatial frequency A.---Multiple MTF values at different spatial frequencies form a set of curves to obtain the MTF curve of the camera.
After mathematical processing, the MTF function (modulation transfer function) of the camera can be obtained

In this case, we need to calculate the response values at different spatial frequencies to calculate the MTF. Should you take multiple pictures? Is this inconvenient?
Then SFR (spatial frequency response) was introduced

SFR (Spatial Frequency Response):
According to mathematical theoretical derivation, people found that only a black and white hypotenuse (knife edge) can be converted to an MTF that is approximately equal to all spatial frequencies.


5. EV (also called illuminance)
EV is the abbreviation of Exposure Values in English. It is an amount that reflects the amount of exposure. It was originally defined as: when the sensitivity is ISO 100, the aperture coefficient is F1, and the exposure time is 1 second, the exposure value is defined as 0, and the exposure value is reduced by one. Stop (the shutter time is reduced by half or the aperture is reduced by one stop), EV+1; the exposure is increased by one stop (the shutter time is doubled or the aperture is increased by one stop), EV-1.
SLR cameras or DCs have automatic exposure functions that accurately detect the light intensity of the shooting environment through their own metering systems. This automatically calculates the correct aperture + shutter speed combination. This way the photo will be correctly exposed.
However, under some special light and shadow conditions (such as backlight conditions), the metering system will not be able to perform correct metering on the subject, so that the photo cannot be properly exposed. At this time, we have to perform +/-EV based on experience and artificially intervene in the camera's automatic exposure system. The result is a more accurate exposure.


6. AWB (automatic white balance)
Automatic white balance is a very important concept in the field of television photography, through which it can solve a series of problems in color reproduction and tone processing.
Many people will encounter such problems when shooting with digital cameras: the images shot in the room with fluorescent lights will appear greenish, the scenes shot under indoor tungsten lighting will be yellowish, and the images shot in the shadow of the sun will appear greenish. The photos I received are inexplicably bluish, and the reason lies in the "white balance" setting.
Automatic white balance is usually the default setting of digital cameras. There is a histogram with a complex structure in the camera, which can determine the white balance reference point in the picture, so as to achieve white balance adjustment. The accuracy of this automatic white balance is very high, but the effect is poor when shooting in low light conditions, such as cloudy weather, the effect of many automatic white balance systems is extremely poor, it may cause a bluish cast.


7. ISO standards (International Organization for Standardization)
ISO standards refer to standards formulated by the International Organization for Standardization (ISO). The International Organization for Standardization is a worldwide federation of national standardization agencies with 140 member countries. According to the organization's charter, each country can only have one of the most representative standardization groups as its members. China's former State Administration of Quality and Technical Supervision participates in ISO activities in the name of CSBTS.


8. ISO (sensitivity)

Sensitivity, also known as ISO value, is a measure of how sensitive a film is to light. It is determined by sensitivity measurement and the measurement of several numerical values. It has recently been standardized by the International Organization for Standardization. For less sensitive films, a longer exposure time is required to achieve the same image as a more sensitive film, so it is often called a slow film. Highly sensitive films are therefore called fast films. Whether in digital or film photography, using a higher sensitivity to reduce exposure time usually results in lower image quality (due to coarser film grain or higher image noise or other factors).


9. light intensity
Light intensity is a physical term that refers to the luminous flux of visible light received per unit area. Abbreviated as illuminance, the unit is lux (Lux or lx). A quantity used to indicate the intensity of light and the extent to which the surface area of an object is illuminated.
Light intensity has a great influence on the photosynthesis of organisms. Can be measured with a light meter.


10. Resolution
Resolution, also known as resolution and resolution, can be subdivided into display resolution, image resolution, printing resolution and scanning resolution.
Resolution determines how fine the details of a bitmap image are.
Typically, the higher the resolution of an image, the more pixels it contains, the clearer the image, and the better the quality of the print. At the same time, it will also increase the storage space occupied by the files.
The units used to describe resolution are: dpi (dots per inch), lpi (lines per inch), ppi (pixels per inch), and PPD (PPPixels Per Degree, pixels per degree). But only lpi is a scale that describes optical resolution. Although dpi and ppi are also units within the resolution category, their meanings are different from lpi. Moreover, lpi and dpi cannot be converted and can only be estimated based on experience.
In addition, ppi and dpi are often used interchangeably. But there are also differences in the fields they use. From a technical point of view, "pixels" only exist in the field of computer display, while "dots" only appear in the field of printing or printing.