Technical principles of infrared cameras

Principle characteristics of infrared cameras

Light is a kind of electromagnetic wave. Usually people divide infrared light into three parts: near, medium and far infrared. Near-infrared refers to the wavelength of 0.75 to 3.0 microns; mid-infrared refers to the wavelength of 3.0 to 20 microns; far infrared refers to the wavelength of 20 to 1000 microns.

Light is an electromagnetic wave that has the same nature as radio waves. Its wavelength range ranges from a few nanometers (1nm=10-9m) to about 1 millimeter (mm). The human eye can only see part of it, which we call visible light. The wavelength range of visible light is 380nm ~ 780nm. The wavelength of visible light is divided into red, orange, yellow, green, cyan, blue and violet from long to short. The wavelength is shorter than violet. It is called ultraviolet light, and the wavelength longer than infrared light is called infrared light. The wavelength of infrared light is between 780nm and 1000μm, which is between radio waves and visible light.

Infrared lamps are divided into two types: semiconductor solid light emitting (infrared emitting diode IR LED) according to their infrared radiation mechanism

Infrared lamps and thermal radiation infrared lamps. The former is more commonly used in CCTV infrared cameras.

The principle of infrared emitting diode (IR LED) infrared light is: a matrix of infrared light-emitting diodes forms a luminous body. The infrared emitting diode is made of a PN junction made of a material with high infrared radiation efficiency (usually gallium arsenide (GaAs)), and a forward bias voltage is applied to inject current into the PN junction to excite infrared light. The spectral power distribution has a center wavelength of 830~950nm and a half-peak bandwidth of about 40nm. It is a narrow-band distribution and is within the range that ordinary CCD black and white cameras can perceive.

The emission power of infrared light-emitting diodes is expressed in irradiance μW/m2. The maximum radiation intensity of an infrared diode is generally directly in front of the optical axis, and decreases as the angle between the radiation direction and the optical axis increases. The angle at which the radiation intensity is 50% of the maximum value is called the half-intensity radiation angle, that is, the half-power angle. The radiation angles of infrared light-emitting diodes with different packaging process models are different.

Infrared lamps have different powers and two wavelengths, 715 and 830nM. The difference in wavelength determines the lighting distance and effect of the infrared lamp:

The 715nM infrared lamp has a long lighting distance. The effect is good, but it will produce red bursts (home digital cameras use this kind of infrared lamp as fill light);

Using 830nM infrared lamps, there is basically no red burst phenomenon or the red burst is very small

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Infrared lamps mainly use infrared emitting diodes on the general market. We introduce their principles and characteristics as follows: the luminous body is composed of a matrix of infrared light-emitting diodes. The infrared emitting diode is made of a PN junction made of a material with high infrared radiation efficiency (usually gallium arsenide (GaAs)), and a forward bias voltage is applied to inject current into the PN junction to excite infrared light. Its biggest advantage is that it can have no red burst at all (using 940-950nm wavelength infrared tube) or only have weak red burst (red burst is visible red light) and has a long life. The emission power of infrared light-emitting diodes is expressed as irradiance μW/m2. In response to this situation, Foxconn's FI-930C and FI-970C fully considered this issue when designing, using high-efficiency light-emitting diodes and a built-in heat dissipation system in the camera to enable the camera's stable working time to reach 25,000 hours!

Infrared ray is a kind of light wave with a wavelength ranging from a few nanometers (nm) to about 1 millimeter (mm). The human eye can only see part of it, which we call visible light. The wavelength range of visible light is 380nm~780nm. The wavelength of visible light is divided into red, orange, yellow, green, cyan, blue and violet from long to short. The wavelength is longer than red light. is called infrared light. The wavelength of infrared light is between 780nm and 1000μm, which is between radio waves and visible light.

Infrared cameras are actually monitoring equipment that integrates surveillance cameras, protective covers, infrared lights, power supply and cooling units, etc. At present, the most commonly used infrared waterproof camera in monitoring projects is active infrared, which emits infrared rays from LEDs. CCD or CMOS can be used to feel the spectral characteristics of infrared light (that is, it can feel visible light and infrared light), and it can be used with infrared lamps as "illumination sources." "Come to night vision imaging. The market is dominated by infrared LEDs emitting wavelengths of 850nm and 940nm. The early infrared night vision systems were not used for civilian purposes, but were mainly used in the military. However, because the active infrared night vision system emits infrared rays and is easily discovered by the enemy, it has been basically eliminated in the military. Infrared thermal imaging cameras (passive infrared cameras) are mainly used in the military field. Their camera systems can sense infrared radiation emitted by objects above absolute zero (-273°C). The higher the temperature of the object, the more infrared rays it radiates and it is easier to image. . Due to target market and manufacturing cost issues, few security manufacturers that produce civilian cameras produce passive infrared cameras.

Infrared light is the core component of infrared cameras. There are two main categories in the market. One is the traditional bead LED light, which is a more traditional and commonly used infrared light source. Later, different " "Piranha" infrared lights can also be divided into traditional infrared lights; the other is the "array" infrared light source that became popular two years ago. Its principle is to integrate many light-emitting chips, and the photoelectric conversion efficiency is higher than ordinary ones. High, the photoelectric conversion rate of ordinary LED is 10%, while the photoelectric conversion rate of "array" infrared camera can be increased to more than 25%, which is why the "array" infrared waterproof camera makes all objects "too white" . At night, faces and clothes are difficult to distinguish in images. In addition, its heat cannot be fixed around the lens of a surveillance camera like traditional infrared lights. Most of the "array" infrared cameras seen on the market have the light board separated from the lens, and the infrared lights are fixed on the upper and lower or left and right sides respectively. side.

The infrared light source is related to the life and distance of the surveillance camera. Generally speaking, the radiation power of the infrared light is proportional to the forward operating current. However, when it is close to the maximum rated value of the forward current, the temperature of the device is affected by the The heat consumption of the current increases, causing the light emission power to decrease. If the current of the infrared diode is too small, it will affect the performance of its radiation power, but if the working current is too large, it will affect its life and even cause the infrared diode to burn out. Therefore, the working current is required to be accurate and stable, otherwise the performance and reliability of the radiation power will be affected. The radiation power will decrease as the ambient temperature increases (including the increase in ambient temperature caused by its own heat). For infrared lamps, especially long-distance infrared lamps, heat consumption is an issue that should be paid attention to when designing and selecting.

Imaging Principle

In night vision monitoring systems, the conventional method is to use visible light illumination, but this method has the disadvantages of not being concealable and easily exposing the monitoring target, so it is rarely used. , concealed and scientific night vision monitoring uses infrared camera technology. Infrared camera technology is divided into passive and active. Passive infrared photography technology takes advantage of the fact that any substance emits infrared rays above absolute zero (-273°C). The higher the temperature of the object, the more infrared rays it radiates. The most typical camera made using this principle is an infrared thermal imaging camera. However, this special infrared camera is expensive and is therefore limited to military or special occasions. Active infrared camera technology uses infrared radiation "illumination" (mainly infrared light), and uses ordinary low-light black-and-white cameras, color-to-black-and-white cameras, or infrared low-light color cameras to feel the infrared light reflected back by the surrounding scenery and environment. Night vision surveillance. Active infrared camera technology is mature and stable, and has become the mainstream of night vision surveillance.

Infrared integrated camera is a camera equipment that integrates camera, protective cover, infrared light, power supply and cooling unit, etc. The basic principle of achieving night vision is to use an ordinary CCD black and white camera to feel the spectral characteristics of infrared light (that is, it can feel both visible light and infrared light), and use the infrared lamp as an "illumination source" to perform night vision imaging. The power and angle of the infrared lamp, the configuration of the camera, the infrared lens with a certain focal length, and whether there is good power supply and heat dissipation are important parameters for judging the performance of the infrared integrated camera.

There are also many products on the market that separate cameras from infrared projectors. This requires users to have sufficient understanding of the performance of infrared lamps and cameras, and to be able to make reasonable adjustments based on the angle of the infrared lamp, camera lens parameters, etc. Match.