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Classification of the CCTV lens and the HD IPC CCTV lens, selection
1 Introduction
IPC HD compared to Station Street Patrol, that CCTV lens is no doubt that the camera "eyes". "Eyes", a completely determined on the camcorder's good or bad. IPC gradually into the HD era, the CCTV lens more deadly effect on the image, are increasingly being perceived by the user!
CCTV lens manufacturers at home and abroad and thousands of CCTV lens types of all kinds, and how to evaluate the quality of the CCTV lens? How to choose the right CCTV lens? This article to discuss all aspects of the CCTV lens working principle, the practical significance of the parameters of the CCTV lens, the CCTV lens selection!
2 CCTV Lens working principle
Speaking of the principle of the CCTV lens would have to mention the geometrical optics three laws: Rectilinear propagation law, the law of refraction, the law of reflection
The light linear law of propagation of the imaging principle (such as a pinhole schematic), a beam of light to reach the surface, the light reflected back to radiate uniformly. The nature of the objects will be sent out light (reflection or spontaneous emission), the CCTV lens is by collecting the reflected light within a region, and then by the refraction of glass or plastic, the beam convergence convergence in a very small area to achieve imaging purpose. And this small area is the imaging area, that is, the imaging plane. Placed Sensor on the imaging surface, such as photographic film, CCD, CMOS imaging of all objects in this region can be obtained.
CCTV lens converging light refracted like light refraction process brings some error: different spectral refractive index, so bring color; inconsistencies brought about by the optical path error in the thickness of the edge of the CCTV lens and the center of the CCTV lens glass, so as to import the aberration of the optical path; difference of the light incident angle of the type of the field of view bring the aberration of the visual field. The quality of the CCTV lens is reflected in the correction of these errors.
3 CCTV lens classification
CCTV lens industry development to the present derived from a variety of classification, introduces some commonly used classification in the IPC industry.
3.1 Interface Type Category
3.1.1 M12 interface trigger CCTV lens
M12x0.5-6g standard threaded connection, typically trigger fixed focus CCTV lens, the individual manufacturers manual zoom board CCTV lens made of such interfaces. M12 CCTV lens generally smaller volume, structure space, and therefore more suitable for demanding applications camera volume, such as elevator UFO Indoor Mini Dome, pinhole. But precisely because of its smaller size also brings a limitation that can not be bigger aperture, are generally small, common market is F1.8, F2.0, which also brought to the M12 CCTV lens relatively large limit!
3.1.2 Ø14-line board CCTV lens
Ø14 & M12 relative aperture can be greatly improved, F1.2, F1.4 is also a common market model. The Ø14 more common in the general short-range zoom CCTV lens, for example, 2.8-12mm, 3-8mm, etc.. Widely used in the hemisphere, infrared cylindrical machines and other products.
3.1.3 CS and C mount CCTV lens
C / CS CCTV lens intercept and interface thread standard interface definition. The threaded portion is the same: 32 thread per inch, large diameter 25.4 trails 24.541 diameter 24.884. C interface intercept 17.526mm, CS for 12.5mm. General camera CS interface, so customers need to pay particular attention to the interface of the match, C mount CCTV lens must be applied to the CS camera plus a CS / C adapter ring. After the intercept mismatch error easily lead to a serious image problem, such as 1 ) for a period of CCTV lens can not focus; 2 ) curvature of field is too large fuzzy edges.
3.1.4 Non-standard interface CCTV lens
Security industry to use more for the movement of the CCTV lens, each manufacturer integrated movement at the interface of the CCTV lens are not the same, the need for camera individually designed according to the specifications of the CCTV lens, and the interface is very high precision camera manufacturers design, process requirements is relatively high, and better able to show the manufacturer's technical strength.
3.2 Sensor size classification
CCTV Lens imaging sensor size, the CCTV lens surface is greater than at maximum can match much Sensor points. Such as 1/3 "CCTV lens, then the the the effective imaging surface diameter greater than 6mm, can match the 1/3 1/4sensor.
3.3 aperture Category
Fixed aperture: the diaphragm as a fixed value, the flux entering the CCTV lens is not controllable, is generally used in a relatively low number of products.
Manual Iris: the aperture manually adjustable, customers need according to the current installation environment adjust the aperture value, and then fixed for conditions. Fixed relative to the scene. Require a higher skill level on the installation and construction.
DC iris: DC motor drive auto iris aperture can be controlled according to the ambient brightness value change into small, so that the camera able to adapt to the different brightness of the scene.
P-iris : P-Iris stepper motor control aperture size, can be precise and accurate control of the iris position to achieve better depth of field effects to enhance image quality.
4 CCTV lens important parameters
4.1 Focal Length (f)
The textbook explanation is the the infinity beam axis that is parallel to the light, the point of convergence of light through the CCTV lens to the distance of the CCTV Lens emperor. Can be simply understood as the degree of convergence of the CCTV lens. Is the most basic and important features of the CCTV lensparameters, the reaction of the observation range of magnification and other important application parameters.
In practical applications, the focal length actually does not make sense, we are more concerned about the focal length reflects the additional information: 1) field of view; 2) a certain distance under magnification.
In addition, the focal length can also reflect its technical difficulty, the longer the focal greater the difficulty to both sides of technology, ultra short-focus, super-telephoto CCTV lens will be relatively expensive.
4.2 field of view (FOV)
The FOV size refers to the angle range of the CCTV Lens can be observed, as determined by the size and focal length of the Sensor. The mainstream Sensor square, it is also sub-horizontal field of view (H), vertical FOV (V), diagonal field of view (D).
FOV formula:
FOV (H) = 2arctan (H/2f)
FOV (V) = 2arctan (V/2f)
FOV (D) = 2arctan (D/2f)
Where H is the horizontal width of the Sensor, V is the vertical width of the Sensor, D Sensor diagonal length, f is the focal length of the CCTV lens. It should be noted that very large (e.g. 3.6,2.8 mm) CCTV lens distortion at the short focal calculated FOV inaccurate, generally much smaller than the actual field of view angle, CCTV lens need The data provided by the manufacturer or the actual test.
4.3 Resolution in pixels (Resolution)
CCTV lens resolution is an important manifestation of the quality of the CCTV lens, the higher the resolution, the stronger the ability of the CCTV Lens can distinguish the details, you can see smaller objects can be simply understood. Resolution of the megapixel CCTV lens process plant is also the easiest problems, each one of the CCTV lens, the size of each structural member subtle deviations may cause undesirable. Therefore, a CCTV lens marked 200w pixels its actual effect is not necessarily good, must go through rigorous testing in order to know whether or not to comply with the design, the resolution really match 200w pixel sensor.
In the discussion of the resolution, we can not ignore the other parameters are closely related with that MTF values.High resolution CCTV lens of a lot of time and not on behalf of its imaging results is good, that is the contrast or contrast may be relatively low. In the evaluation, selection of a CCTV lens will be more concerned about the MTF curve.
The MTF curves describes the parameters of a camera image quality, which contains different field of view resolution and contrast information.
The abscissa is the value of the spatial frequency resolution, i.e. the details of the resolution capability; ordinate MTF value, its response is the contrast transfer effects, i.e. the contrast of the image effect. High resolution, MTF value, CCTV lens imaging results will be good.
4.4 FNO
FNO is the ratio of the focal length and aperture, the reaction to the amount of light through the CCTV lens. FNO smaller the value, the larger the aperture, the CCTV lens the larger the amount of the light source. Generally will choose the FNO small CCTV lens in low illumination environment. The FNO small of course, will lead to other problems, the most obvious in bright light storm shorter depth of field, decreased sharpness of the image periphery. Scene during the day will generally close the aperture to achieve better depth of field and clarity.
4.5 Depth of field
The depth of field is in the focus plane before, after each period of the spatial extent of the image is clear. Distance determined by to diffuse plaque size, CCTV lens focal length, focus distance, FNO parameter determines.
Diffuse plaque is smaller, the shorter the depth of field. Diffuse plaque parameters Sensor, Sensor resolution is the higher, the smaller the dispersion spots, so the more high-definition camera to its dispersed spot smaller, the shorter the depth of field, so to get a better depth of field in HD IPC on the CCTV lens and the CCTV lens and Sensor with more demanding.
The longer the focal length, the shorter the depth of field; focusing distance farther, the longer the depth of field; larger diameter, the shorter the depth of field; For a CCTV lens, designed after the basic shape determines the level of depth of field, CCTV lens choice particularly important.
When the megapixel CCTV lens and Sensor have been stereotyped To better depth of field, require customers to choose the focusing distance, the reasonable control of the aperture.
4.6 IR correction
The refractive index of the different spectra of the same material is inconsistent, visible effect over a period of time, so the CCTV lens is also only for visible light (390 to 700nm) color correction to infrared are virtual focus the. Recently, the application of infrared light is becoming increasingly popular, especially into the HD era greatly reduce the depth of field makes the IR focus problem was more and more customer focus. The IR CCTV lens infrared and visible light correction, making it the focal plane essentially coincident focus during the day, night IR On clear.
5 CCTV lens Industry Situation and Development Direction
5.1 CCTV lens development direction
5.1.1 High-resolution
Sensor resolution is getting higher and higher, higher and higher demands on the CCTV lens. IPC has entered 720P, 1080P high-definition era, the manufacturers of the new CCTV lens are basically to meet 2Mega, 3Mega design. 1080P period of time will be a mainstream CCTV lens should also be the main 2MP, 3MP; ITS applications, we believe that 2MP, 5MP CCTV lens will be mainstream.
5.1.2 Electric / Auto Zoom / Focus
Into the HD era, the camera depth of field is shorter, smaller depth of focus, the focus becomes increasingly difficult when installation is difficult to find the right focus, it is difficult to determine the focus is intact, the camera does not embody the best results. Once the camera is installed, the stress changes of the structural member (even small changes to 0.02mm) may also lead to the change of the image, This requires the camera can focus automatically or remote focus within the base station. Automatic / electric Focus is not technically difficult, but the rising cost, I believe that technology continues to progress its cost will be a corresponding reduction in the auto / electric CCTV lens will be the next stage of development, will probably become the standard .
6.0 HD IPC CCTV lens selection principles
The reasonable and appropriate choice of parameters of the CCTV lens, grasp the trade-offs and balance: a) Resolution: Resolution is not as high as the higher the CCTV lens resolution is much higher than the Sensor has been limited by the Sensor, which is a cost waste; resolution in addition to the center of attention, but more importantly is the need to focus on the peripheral field of view, in terms of the camera center are easily able to meet the requirements, but around performance is vastly different. b) FNO: aperture can reflect the amount of light through the CCTV lens, but not all, with the respective wavelength transmittance of each one CCTV lens of the lens, so the need to focus the CCTV lens of the overall amount of light. Fno often with the resolution, contrast is a contradiction, the large diameter of the CCTV lens is often the resolution will reduce the contrast of the screen will decline; in order to get a better resolution and contrast, at the request of the cost of a lot of CCTV lens manufacturers have chosen to sacrifice light through caliber to achieve. Aperture and resolution ascend together, the cost will rise sharply. Selection need to weigh the scene, the application model, in order to get a relatively balanced. c) the size of the image plane (image format): selection requirements CCTV lens imaging surface, not the bigger the better relative Sensor Sensor matching. Sometimes use 1/2 "or 2/3" CCTV lens to match the 1/3 "Sensor to achieve better image uniformity, but this is not necessarily good, there may be some peripheral illumination, resolution rate becomes low risk d) Focal Length: appropriate choice of focal length range according to the scene being monitored and the need to see the object, the installation location e) IR: IR corrected CCTV lens to achieve, you need to use some ultra-low dispersion precious materials and design, the processing difficulty greatly enhance a sharp increase in the cost of a reasonable choice, so need to use scene.
Of the parameters of the CCTV lens there is a lot of conflicting constraints, the rise of one of the parameters is often associated with the reduction of another parameter, and therefore need to use needs and focus, a reasonable trade-off in order to get a relatively balanced effect. For example, the aperture increases to low illumination effect can be better, but this often requires sacrificing the resolving power of the CCTV lens, bring the surrounding image effect is reduced, the overall picture contrast reduction. To enhance the quality and balance of the cost parameters in a breakthrough, the rising cost is not linear, often quality increased slightly to bring the price doubled, and therefore needs to consider enough on the line!
The CCTV lens manufacturers indicated parameters is not entirely credible: CCTV lens industry was mixed, there is a lot of focal length, FNO, IR, and other parameters of the phenomenon of virtual standard, seems to be the industry 'hidden rules'. Therefore, to choose a suitable CCTV lens, the CCTV lens must be professional, system testing, evaluation, in order to eliminate the false and retain the true to the customer better quality, which is the meaning of existence Uniview CCTV lens recommended list.
Not the excessive superstition of Japanese CCTV lens: Day series CCTV lens has been price high prices, and domestic, Taiwan-based CCTV lens continuous rise in many ways already can replace Japanese products, especially the SD CCTV lens, basic has not weaker than the Japanese, and the price was relatively low In some applications, switch to the Department of Taiwan, domestic CCTV lens is also very good choice; Japanese CCTV lens is not each manufacturer for each model are good, they also design error, process error, and therefore the need for system evaluation test, all the data to speak!
A good image of the CCTV lens effect is not necessarily good: good CCTV lens just that there was a good image source image source can show to the user a good feel of the screen, was kind enough to follow-up image acquisition, image processing, image transmission, image display and other aspects are closely related.
IPC imaging process as shown above, the effect of user experience is determined by the entire process of the short board. CCTV lens, Sensor, ISP most central, three kinds of co-ordination between 2MP CCTV lens match SD Sensor, Sensor resolution is low, such as the use of that image effects can only have the effect of SD. Sensor image signal is relatively poor, requires ISP to do post-processing, including interpolation, de-noising, go dead pixels, edge sharp, white balance and other operations, you can make the images more delicate, closer to the real world the image. Other aspects also have a greater impact on the image filter OLPF, the high-frequency part of the CCTV lens is filtered, degradation of resolution. The median image acquisition is too little detail is lost, the loss of image effect. Encoding, transmission poor lose data. The display device can not restore the true image.
7 How to determin a good CCTV lens
7.2 Coating of CCTV lens.A good CCTV lens has multi-layer coating with several colors, like green coating, red coating, blue coating etc. A good CCTV lens you can see above colors at different direction of light, that is called multi-layer coating; a normal coating will reflect single green, single blue or single red etc. A good coating will bring imaging sharp and bright.
7.3 Aperture of CCTV lens.A big aperture (small F.No) is difficult to make and is a good CCTV lens. Big aperture will have good imaging at dark area or night vision, but a low level CCTV lens will be smaller aperture, which will lead to horrible surveillance without lighting at night.
8 Conclusion
From the above analysis it can be seen that the CCTV lens within the security industry classification and direction of development, as well as our point of concern in the selection of a CCTV lens. All in all, a good shot is a good first step in the image, is the cornerstone, only the cornerstone of a solid enough be possible to have a high quality of IPC.