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In 1992, the American National Standards Institute (ANSI) developed a standard numbered IT7.215 for measuring the luminous flux of projectors. This is the ANSI lumen method. The luminous flux value measured by this method is ANSI lumens. The specific measurement details of IT7.215 cannot be viewed for free (paid, 25 US dollars). According to the information disclosed by others, it is to project a picture that is evenly divided into 6 grids on a fixed area (60 inches diagonal, 4:3 ratio) of the screen at a fixed distance of 2.4 meters and a room temperature of 25 degrees. The 6 grids are 0%, 5%, 10%, 90%, 95%, and 100% white rectangles to adjust the projection contrast. When all 6 grids can be clearly recognized by the human eye, a pure white image (generally called 100% full white field signal) is used. At this time, the screen is divided into 9 areas, and the illumination at the center of each of the 9 areas is measured and the average value of the 9 is taken. Then the result is multiplied by the total area to finally get the value of the luminous flux (generally called light output, translated into Chinese as light output). The key point here is that the ANSI method directly measures not luminance or brightness, but illuminance. The luminance is multiplied by the area to calculate the luminous flux, and then the luminous flux value, i.e. the lumen value, is marked in the brightness column according to the requirements of the FTC mentioned above, which gives the so-called "brightness" of the projector. This is actually the application of Lambertian photometry. In 1998, after the IT7.215 standard had been used for 6 years, ANSI updated and supplemented it and established the new IT7.227 standard (IT7.228 for fixed-resolution projection, 227 is our common variable-resolution projection) to replace the previous IT7.215. For example, the preset adjustment chart used to prevent cheating has an additional 85% and 15% white grid, with a total of 8 grids, and there are intervals in the middle and around the upper and lower columns, and it no longer covers the entire screen. ANSI has not updated it since then.

Luminous flux, luminous flux, is the power of visible light that can be felt by the human eye. The unit is lumen, symbol lm. Luminous intensity, luminous intensity, is the luminous flux emitted by a light source in a given direction and per unit solid angle. The unit is candela, symbol cd. Illuminance (to distinguish irradiance, also called illuminance), illuminance, is the luminous flux received per unit area. The unit is lux, symbol lx. Brightness (to distinguish radiance, also called luminance), luminance, is the luminous flux emitted per unit surface area and per unit solid angle by the orthographic projection of visible light on a plane perpendicular to its light transmission direction. The unit is candela per square meter, symbol cd/m^2, formerly called nit, synonymous. There is also Xiti, which changes square meters to square centimeters, a 10,000-fold relationship. Lambert, lambert, named after Lambert himself, 10,000 times candela per square meter divided by π, symbol L. It is 10,000 times greater than the above luminance (that is, π). The extra π is for the convenience of calculation in some occasions. In essence, it is still brightness. The metric meter in the unit was later replaced by the imperial unit of feet, so there was the "foot-lambert" unit, with the symbol written as fl or ft-L. Convert metric meters to feet and then divide by π, and finally 1ft-L is about 3.43 times cd/m^2, and 1L is about 929 times ft-L. Although it is rarely used in projection, foot-lambert is often used in scenes where imperial units are used, such as exhibition halls and theaters.

There are two common types of home projectors: single DLP and 3LCD. Single DLP projection is a time-sharing reflective projection that uses a single-channel DLP chipset from Texas Instruments (TI), while 3LCD is a simultaneous transmissive projection with three LCD panels represented by Japanese companies such as Epson. The internal structure of a 3LCD projector is relatively complex, so let's briefly introduce the principle. The light source is first polarized to convert all transverse waves in the light source into longitudinal waves (because the liquid crystal it uses can only pass longitudinal waves), and then the light is evenly distributed through an integral lens array. Next, it passes through a dichroic mirror (also called a dichroic mirror) to produce red light and its complementary color cyan light. The red light is reflected by the mirror, and the cyan light is then decomposed into green light and blue light by a second dichroic mirror, thus forming three beams of RGB light. The red and blue lights are reflected by the corresponding mirrors (green light itself is reflected, so no mirror is needed) together with the green light, respectively, to three light-transmitting LCD screens that intersect at right angles (typically, the HTPS LCD screen used by Epson has strong light transmittance). A reflective array that shortens the optical path of the blue light and a dichroic prism (Figure 8) are also needed. Finally, the light formed by the three LCD screens is converged to the lens through the dichroic prism and then projected onto the screen. Figure 8 Demonstration of color separation prism Green light needs to be transmitted, and red and blue light are incident vertically and need to be reflected, so ordinary reflectors cannot be used directly here. Only color separation mirrors can be used to form a prism, which reflects red and blue light while transmitting green light to synthesize colors. Therefore, color separation prism is based on its principle; and some people also call it color combination prism, which is based on its purpose. It seems to contradict each other, but it is actually the same thing. It is estimated that some people still do not understand why the last prism is called color separation prism when it is obviously used to synthesize colors. You may as well think that it can be used in reverse, so either name is probably fine, of course, the former name is generally used. Single DLP type projection is relatively simpler. After passing through the laser, the light source is incident on the filter of the color wheel, at which time the RGB three-color light is formed in time division, and then evenly distributed through the integrator (some models use relay mirrors instead of integrators to reduce costs and volume) and incident on the DMD chip after passing through the TIR prism (Total Internal Reflection Prism). There is a square area on the DMD chip covered with tiny aluminum reflectors at the pixel level, which are reflected to the lens group and finally projected onto the screen. By controlling the rotation angle of the reflector on the DMD through the circuit, different grayscales can be obtained, and then combined with the RGB color generated by the filter, the color is superimposed using the visual residual characteristics of the human eye, and finally the color picture we see is formed. This is the working principle of single-chip DLP. In fact, there are also high-end models that use 3 DMD chips at the same time. Because they are not included in the common consumer-grade models and the internal structure is too complicated, they are not involved here. Therefore, the reason why the internal structure of a single DLP is simple is not only because there is only one DMD chip instead of three LCDs, but the most important thing is that the most complex structure and the most core functions are all handled by the DMD chipset itself, so when we disassemble it, we will feel that DLP is much "simple". But in fact, the DMD chipset is very complex. Due to the limited space of this article, it is expected that the chipset will be placed in the third article. In addition, there are also many types of light sources for single DLP projection, including three-color LED, single/dual-color laser + phosphor (generally called Laser-phosphor), three-color laser (RGB laser), and the most common ultra-high pressure mercury lamp (such as Philips UHP) bulb light source. In addition, there are also mixed types of light sources, such as laser + LED light source, or UHP + UHP dual light source superposition, etc.

ISO (International Organization for Standardization) and IEC are both international organizations, and there is a cooperative relationship between the two, so the standards they set also have the nature of mutual reference. On August 26, 2005, three years after IEC released 61947-1, ISO and IEC jointly released the ISO/IEC 21118:2005 standard, which has only 15 pages, which is half of the 30 pages of IEC at that time. Of course, the main reason is that the other standards cited in it are not described in detail, but only the numbers are written. Subsequently, ISO21118 became the standard used worldwide after ANSI IT7.227 (Japan passed the domestic implementation of ISO standards the following year, so Japanese products will be marked with ISO lumens). After several iterations and updates, as of the time of writing this article (December 31, 2021), the latest version of the standard is the 2020 version released on February 1, 2020, namely ISO/IEC 21118:2020 (paid, 118 Swiss francs). ISO21118 mainly includes five major tests, namely light source, audio output, background noise, maximum power consumption, and standard power consumption. The light source part contains several small items, such as "brightness", contrast, and uniformity. The test of the "brightness" part still continues to use the test method in IEC 61947-1, and the pre-adjustment chart also uses the 8-grid chart, but only some adjustments are made to the individual test environment. For example, in the 1990s, a three-gun CRT projector at a distance of 2.4 meters could only project a 60-inch screen, and the projection ratio was only 2.4m/0.74m=3.24. Today, it is unimaginable that the projection ratio of household telephoto projectors is generally 1.1-1.7. Therefore, the ISO standard sets adjustable ranges for the size and area of ​​the screen and the distance according to the projection ratio, but the overall test method itself has not changed, and it is still the 9-point illumination average method of ANSI, so the ISO lumen test results will not have a large difference from ANSI in terms of values.

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ANSI Standard In 1992, the American National Standards Institute (ANSI) developed a standard numbered IT7.215 for measuring the luminous flux of projectors. This is the ANSI lumen method. The luminous flux value measured by this method is ANSI lumens. The specific measurement details of IT7.215 cannot be viewed for free (paid, 25 US dollars). According to the information disclosed by others, at a fixed distance of 2.4 meters, at a room temperature of 25 degrees, after preheating for 15 minutes, a picture that covers the entire screen and is evenly divided into 6 grids is projected on a screen of a fixed area (60 inches diagonal, 4:3 ratio). The 6 grids are 0%, 5%, 10%, 90%, 95%, and 100% white rectangles to adjust the projection contrast. When all 6 grids can be clearly recognized by the human eye, a pure white image (generally called a 100% full white field signal) is used instead. At this time, the screen is evenly divided into 9 areas, and the illumination at the center of each of the 9 areas is measured and the 9 average values ​​are taken. The result is then multiplied by the total area to obtain the value of the luminous flux (generally called light output, translated into Chinese as light output). The key point here is that the ANSI method directly measures not luminance nor brightness, but illuminance. The luminance is multiplied by the area to calculate the luminous flux, and then the luminous flux value, i.e. the lumen value, is marked in the brightness column according to the requirements of the FTC mentioned above, which gives the so-called "brightness" of the projector. This is actually the application of Lambertian photometry. In 1998, after the IT7.215 standard had been used for 6 years, ANSI updated and supplemented it and established the new IT7.227 standard (IT7.228 for fixed-resolution projection, 227 is our common variable-resolution projection) to replace the previous IT7.215. For example, the preset adjustment chart used to prevent cheating has an additional 85% and 15% white grid, with a total of 8 grids, and there are intervals in the middle and around the upper and lower columns, and it no longer covers the entire screen. ANSI has not updated it since then. ANSI has stopped, but other standards are ready to move.