Optical knowledge in the camera module (1)

First a brief understanding of the concept of the Camera Module we want to talk about, the camera module, the full name of the Camera Compact Module, abbreviated as CCM, is an important electronic device for image capture.

1. TCP (correlation color temperature)

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

2, TCP (color temperature)

Color temperature is a unit of measurement that indicates the color component of a light. Theoretically, blackbody temperature refers to the color of the absolute blackbody after heating from absolute zero (-273 ° C). After the black body is heated, it gradually turns from black to red, yellow, white, and finally emits blue light. When heated to a certain temperature, the spectral component of the light emitted by the blackbody is called the color temperature at this temperature, and the unit of measurement is "K" (Kelvin).

If the light emitted by a light source contains the same spectral composition as the light emitted by a blackbody at a certain temperature, it is called a certain K color temperature. If the color of the light emitted by a 100W bulb is the same as the color of the absolute blackbody at 2527 ° C, then the color temperature of the light emitted by this bulb is: (2527+273) K=2800K.

In fact, there is no difference between the two meanings of TCP above, generally called color temperature, don't be too tangled

3. MTF (Modulation Transfer function)

Modulation Transfer Function (modulation transfer function) is a relatively scientific method of analyzing the image resolution of a lens. Recently, more and more people have found that although it is a standard, some images can be measured by non-standardization, so MTF is only a reference value, not the whole.

This way of measuring optical frequency is measured by how many lines can be presented in a range of mm, and its units are expressed in line/mm. Therefore, when a lens can achieve the degree of entry and exit, it means that this lens is the so-called perfect lens, but because the design of the lens often has many factors, it is impossible to have such an idealized lens.

MTF cut off(MTF cut-off frequency) : OQAS parameter, indicating that the human eye MTF curve will reach the resolution limit when the spatial frequency reaches the frequency value, that is, the MTF value tends to zero.

Normal people ≧30c/deg, the greater the value, the better the visual quality.

4. SFR (Spatial Frequency Response)

There are many units of spatial frequency (5), of which cycles/pixel (cycles per pixel) is the imaging system in the digital camera.

SFR is the abbreviation of Spatial frequency response, which refers to the amplitude response of a system output relative to the spatial frequency of the input. SFR is similar to the MTF (modulation transfer function) of traditional optical systems, which can intuitively determine the image resolution ability of the system.

The International Standardization Organization (ISO) has developed and standardized the relevant definition and test method of SFR

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 that a vibration or oscillation is completed per unit time;2. the number of times that something happens or a process is completed in a given time]

The dividend here is in units so it's time frequency.

But what if we changed the dividend to the space unit of millimeters? It's the number of times something appears in a millisecond. Here we call it spatial frequency.

[Official: But the spatial frequency is commonly expressed as "line pairs per millimeter" (LP/mm), which is how many line pairs per millimeter width.]

[It can also be expressed in these terms: and the unit of spatial frequency is generally

Line pairs per millimeter (lp/mm),

cycles per millimeter (cycles/mm),

cycles per pixel (cycles/pixel),

LW/PH Line Widths per Picture Height,

Line pair high per image (lp/ph). ]

lp/mm is the most used unit at present. cycles/pixel is an 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 do MTF and SFR do? This is where the resolution of the camera comes in.

The analytical power of the camera is to analyze the effect of images taken by the camera with different spatial frequencies. Different cameras at high spatial frequencies, whose picture is clearer, whose analytical power is higher.

MTF(Modulation Transfer Function):

How to calculate the modulation transfer function of a camera :(a function is obtained based on the analysis of 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 image of the camera at the spatial frequency A is read its brightest value and darkest value)

The calculation formula is:

MTF = (maximum brightness – minimum brightness)/(maximum brightness + minimum brightness)

The MTF value obtained is the resolution force of the camera at the spatial frequency a. —- The MTF curve of the camera can be obtained by forming A group of curves of MTF values at different spatial frequencies.

The MTF function (modulation transfer function) of the camera can be obtained by mathematical processing.

So what that means is that when we calculate MTF we need to calculate the response at different spatial frequencies, does that mean that you should take multiple pictures? Isn't that inconvenient?

And then we have SFR(spatial frequency Response)

SFR(Spatial Frequency Response):

According to mathematical theory, it is found that only a black and white hypotenuse (knife edge) can be converted to approximately equal to all spatial frequencies under the MTF.

EV (also known as illuminance)

EV is an abbreviation of English Exposure Values, which 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 is defined as 0, the exposure is reduced by one gear (the shutter time is reduced by half or the aperture is reduced by one gear),EV+1; Increase exposure by one step (double shutter time or increase aperture by one step), EV-1.

effect

SLR cameras or DC have automatic exposure function, through their own metering system to accurately detect the light intensity of the shooting environment. The correct aperture value + shutter speed combination is calculated automatically. So that the photo can be correctly exposed.

However, under some special light and shadow conditions (such as backlight conditions), the photometry system can not measure the subject correctly, so that the photo can not be correctly exposed. At this time, we have to follow the experience of +/-EV, human intervention in the automatic exposure system of the camera. For a more accurate exposure.

Six, AWB (Automatic white balance)

Automatic white balance is a very important concept in the field of television photography, which can solve a series of problems in color reproduction and tone processing.

Many people in the use of digital cameras will encounter such a problem: the image taken in the fluorescent room will appear green, the scene taken under the indoor tungsten light will be yellow, and the photo taken in the daylight shadow is inexplicably blue, the reason is that the "white balance" setting.

Automatic white balance is usually the default setting of digital cameras, and the camera has a complex rectangular diagram that determines the white balance reference point in the picture to achieve white balance adjustment. The accuracy of this automatic white balance is very high, but the effect is poor when shooting under low light conditions, such as cloudy weather, many automatic white balance systems are very poor, it may cause blue.

7. ISO Standards (International Organization for Standardization)

ISO standards are standards developed by the International Organization for Standardization (ISO). The International Organization for Standardization (ISO) is a worldwide federation of national standardization bodies with 140 member countries. According to the constitution of the organization, each country can only have one of the most representative standardization bodies as its members, and the former State Bureau of Quality and Technical Supervision of China participates in ISO activities in the name of CSBTS.

8. ISO (speed)

Sensitivity, also known as ISO value, is a measure of the sensitivity of a film to light, which is determined by sensitivity measurement and measurement of several values, and has recently been standardized by the International Organization for Standardization. For less sensitive negatives, a longer exposure time is required to achieve the same image as for more sensitive negatives, so they are often referred to as slow negatives. Highly sensitive negatives are therefore called fast negatives. In both digital and negative photography, the use of relatively higher sensitivities to reduce exposure time usually results in lower image quality (due to thicker film particles 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. Illuminance, unit Lux (Lux or lx). A quantity used to indicate the intensity of light and the extent to which an object's surface area is illuminated.

Light intensity has great influence on photosynthesis of organisms. It can be measured by an illuminometer.

10. Resolution

Resolution, also known as resolution, resolution, can be subdivided into display resolution, image resolution, print resolution and scan resolution.

intro

Resolution determines the level of detail in a bitmap image.

In general, the higher the resolution of the 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 files.

unit

The units used to describe the resolution are: dpi (points Per inch), lpi (lines per inch), ppi (pixels per inch), and PPD (PPPixels Per Degree). But only lpi describes the scale of optical resolution. Although dpi and ppi are also units of resolution, they have a different meaning than lpi. Moreover, lpi and dpi cannot be converted, and can only be estimated by experience.

In addition, ppi and dpi are often mixed. But there are differences in the fields they use. From a technical point of view, "pixels" exist only in the field of computer display, and "points" only appear in the field of printing or printing.