“Line”, “progressive” and “interlaced”
In traditional CRT analog TV, the scan of an electron beam in the horizontal direction is called “line”, or “line scan”.
Each frame of the TV is composed of several horizontal scanning lines. The PAL system is 625 lines/frame, and the NTSC system is 525 lines/frame. If all the lines in this frame are continuously completed from top to bottom line by line, or the scanning sequence is 1, 2, 3, …, 525, this scanning method is called progressive scanning.
In fact, one frame of ordinary TV needs to be completed by two scans. The first pass scans only odd-numbered lines, that is, lines 1, 3, 5, …, 525, and the second pass scans only even-numbered lines, that is, lines 2, 4, 6, …, 524. This scanning method is interlaced scanning. A picture containing only odd or even lines is called a “field”. The field containing only odd lines is called “odd field” or “top field”, and the field containing only even lines is called “even field” or “bottom field”. That is, an odd field plus an even field equals one “frame” (one image).
Illuminance is a unit that reflects light intensity. Its physical meaning is the luminous flux irradiated on a unit area. The unit of illuminance is the number of lumens (Lm) per square meter, also called Lux: 1Lux=1Lm/square meter. In the above formula, Lm is the unit of luminous flux, which is defined as the amount of light radiated by pure platinum at the melting temperature (about 1770 ° C), its surface area of 1/160m2 within a solid angle of 1 steradian.
In order to have a perceptual understanding of the amount of illumination, let’s take an example to calculate. A 100W incandescent lamp has a total luminous flux of about 1200Lm. If it is assumed that the luminous flux is evenly distributed on the hemisphere, the illuminance values at 1m and 5m away from the light source can be obtained according to the following steps: the area of a hemisphere with a radius of 1m is 2π×12=6.28m2, and the illuminance value at a distance of 1m from the light source is: 1200Lm/6.28m2=191Lux; similarly, the area of a hemisphere with a radius of 5m is: 2π×52=157m2, and the illuminance value at a distance of 5m from the light source is: 1200m/157m2=7.64Lux. It can be seen that the illuminance emitted from the point light source obeys the inverse square law.
1Lux is approximately equal to the illuminance of 1 candle at a distance of 1m. The minimum illuminance (Minimum Illumination) common in the camera parameter specification means that the camera can obtain a clear image only under the marked Lux value. The smaller the value, the better, indicating that the sensitivity of the CCD is higher. Under the same conditions, the illuminance required by a black-and-white camera is much less than 10 times lower than that of a color camera that still has to deal with color intensity.
What is IRE
IRE is the abbreviation of Institute of Radio Engineers. The video signal unit formulated by this institution is called IRE. Now, the IRE value is often used to represent different picture brightness. For example, 10IRE is darker than 20IRE, and the brightest level is 100IRE. So, what’s the difference between setting the absolute black level to 0IRE and 7.5IRE? Due to the limited performance of the early monitors, in fact, the areas where the brightness is lower than 7.5IRE on the screen basically cannot display the details, and it looks like black. By setting the black level to 7.IRE, some signal components can be removed, thereby simplifying the circuit structure to a certain extent. However, the performance of modern monitors has been greatly improved, and the details of the dark parts can be displayed well. At this time, setting the black level to OIRE can reproduce the picture perfectly.