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Understanding Camera Resolution

What Is Resolution

What are "Effective Pixels"?

What is "Lux"?

About "Infra Red Illumination"

Can Cameras Really See in The Dark Without IR

Why Sony Super HAD CCD II?

 

 

 

What is Resolution?

We've all been in an electronic store where they have an array of plasma and LCD's mounted side by side. We hear the term "higher resolution" in a sales pitch. When we refer to "resolution", it simply means the "overall picture sharpness" and is measured by counting both horizontal lines (vertical resolution) and vertical lines (horizontal resolution) of an image. Refer to the diagram below. The white lines are also counted as a line in the picture. In other words, resolution quantifies how closely the lines are to each other and still be visibly counted. The higher the number of TV lines, the higher the resolution and the more details we can see.

 

 

 Vertical Resolution

 Horizontal Resolution

Close-up of a typical CRT color Screen

 

 

However, when we are talking about "resolution" in terms of a CCTV camera, we only refer to the "horizontal resolution". The number of scanning lines used in a TV system limits the vertical resolution. We cannot count more than 625 horizontal lines in a CCIR/PAL system or 525 horizontal lines in an EIA/NTSC (North American Standard) system. Horizontal resolution is a little bit of a different story. The number of horizontal elements that can be captured by a camera and reproduced on a monitor’s screen also defines the horizontal resolution. And, similar to what we said about the vertical resolution, the horizontal tells us how many vertical lines can be counted. One thing is different; however, because of the TV aspect ratio of 4:3, the width is greater than the height. So, to preserve the natural proportions of the images, we count only the vertical lines of the width equivalent to the height (i.e., three-quarters of the width). This is why we do not refer to the horizontal resolution as just lines but rather TV lines (TVL), and this is why a camera manufacturer only indicates the resolution as a horizontal resolution. For example, if a camera manufacturer says that their camera supports 550TVL resolution, it means that the camera outputs its TV picture to support a monitor with the resolution of 550 vertical TV lines. Remember, the resolution of the camera is only as good as the resolution output of the monitor.

CCTV cameras can be categorized into two groups: high-resolution cameras and standard resolution cameras, in terms of CCD resolution. The CCD resolution is different from the camera resolution. If you check a camera specification sheet, you might find specifications named "Number of Total Pixels" or "Number of Effective Pixels". These specifications tell you what resolution CCD is used in that camera. In general, a camera using a CCD with the resolution of 380K (768 x 494) effective pixels is a high-resolution camera. If a camera uses a CCD with a resolution of 250K (510 x 492) effective pixels, it is considered a standard resolution camera. The high resolution (380K effective pixels) CCD is used to make a camera with 480 TVL resolution and above. On the other hand, a camera that supports a lower resolution than 480TVL, uses a CCD with 250K effective pixels. Thus, you can say that there might be a considerable difference in sharpness between 420TVL and 480TVL cameras. If you see a camera, which uses a CCD with 250K effective pixels but is advertised as 480TVL resolutions or above, it would be wise to confirm the actual resolution. Otherwise, you will end up paying more than you should.

Now that you have an understanding of what resolution means, some of you might wonder how we can count the TV lines to determine the degree of resolution. There are several industrial standards to test the degree of resolution, such as IEEE-208, EIA-1956, CCTV Labs Test, etc. CCTV manufacturers perform resolution tests based on these standards.

How To Test Camera Resolution 

You can test the camera resolution only if you have proper testing equipments and know how to set it up and test it. The testing equipment is normally very expensive, and the standards require you to have professional electronics knowledge to understand it. If you would like to perform a simple test, you can refer to the following test instruction document from CCTV Labs. 

http://www.ktnconline.com/ktncpartner/cctv_test_chart_instructions_02082010.pdf

References: (1,2)
1. CCTV Networking and Digital Technology (2nd Edition) by Vlado Damjanovski, Page 122 - 124
2. CCTV Camera Resolution Test Instruction Document by "CCTVLabs"

 

 

What are "effective pixels"? 

In the last topic on resolution and we had briefly mentioned effective pixels. The following will provide more detail on the definition of "effective pixels". Before we explore the meaning of effective pixels, we need to set the foundation by reviewing the general CCD structure.

 

What is "CCD"?

The term "CCD" is short for "Charge-Coupled Device", which is made of a two-dimensional array of metal-oxide-semiconductor (MOS) capacitors (photosensitive cells). The MOS capacitor is a device that is very sensitive to light (protons). An image can be interpreted in terms of light. In fact, you see an image through your eyes by sensing a different level of light reflected from an object. The MOS capacitors can detect the amount of light (protons) and convert it to the form of electrical charges (electrons). The collected charges are moved into the CCD circuit by using the charge-coupling method. CCD is used in digital imaging applications, such as a digital still cameras and CCTV cameras.

 

What is "Pixel"?

In a digital picture, the term "Pixel" represents "Picture Element", the smallest block of an image or picture which can be expressed or displayed on a monitor screen. As you see on the 3200x close-up image below, the image is made of lots of rectangular blocks and each block is called a pixel. The more pixels within a size-limited picture, the higher the resolution is.

In a CCD, the term "Pixel" refers to a photosensitive cell since each cell gathers image information as a picture element. Thus, similarly to the digital picture, the more photosensitive cells in a CCD, the higher resolution the CCD can achieve.

 

 

What are the "Effective Pixels"?

According to the definition listed on Standard "DCG-001-Translation-2005", effective pixels are the pixels on the image sensor (CCD), which receives input light through the optical lens, and which are effectively reflected in the final output data of the image. Simply, effective pixels are the pixels, which are actually used to produce the final image. The remaining unused pixels are not counted in the Effective Pixel count, but rather are found in the Total Pixel specification. 

Refer to the figure below which describes the pixel-related definitions. The rectangular box including the optical black area is a CCD. In the figure, you can see the image circle of a lens. That circle can be adjusted by changing the focal length of a lens. The optical black area is not used. Area C is also a part of the effective pixels but an opto-electric conversion element vendor does not guarantee this area. Thus, the maximum number of effective pixels guaranteed by a CCD manufacturer would be up to Area B. Area A is the number of effective pixels limited by an image circle of a lens and this area can be expanded up to Area B by adjusting the focal length of a lens. If the image circle of a lens is bigger than Area B, Area A will be limited by Area B.

 

Fig1. Pixel-Related Definitions

  • Area A: Number of effective pixels limited by an image circle of a lens
  • Area B: Maximum number of effective pixels guaranteed by an opto-electric conversion element vendor
  • Area C: Maximum number of effective pixel on a CCD
  • Area D: Number of total pixels on a CCD

 

A CCD manufacturer discloses both the total pixel and effective pixel specifications, and most camera manufacturers use the specs for their own purpose. However, some camera manufacturers will only indicate the number of total pixels (or can be described as "number of pixels") on their specification sheet. This is so they can use it to their advantage when marketing their product. This issue has been causing some confusion among consumers when comparing different brands of the same product.

As we discussed in the previous issue, CCTV camera manufacturers predominantly use a CCD with 250K (510H x 492V) effective pixels to make a CCTV camera under 480TVL resolution and a 380K (768V x 494H) effective pixel CCD to make a 480TVL or higher resolution camera, unless they explicitly indicate that they are using a special CCD.

Now that you have a better understanding of what an effective pixel is and the difference compared to total pixel, you can review a camera spec sheet for accuracy and judge for yourself the integrity of the product.

For further details on pixel-related definitions, you can refer to Standard "DCG-001-Translation-2005", which you can find on the following official website:

CIPA DCG-001: http://www.cipa.jp/english/hyoujunka/kikaku/cipa_e_guideline.html

References: (3,4)
3. CCTV Networking and Digital Technology (2nd Edition) by Vlado Damjanovski
4. Guideline of the Camera & Imaging Products Association (CIPA DCG-001-E-2005)

Standards:
1. CIPA DCG-001-Translation-2005
2. ANSI/I3A IT10.7000-2004

 

What is "Lux"?

The lux (symbol: lx) is the International System of Unit (SI unit) of illuminance and luminous emittance. The SI unit is the modern form of the metric system and is generally a system of units of measurement.

Lux is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface.

In the CCTV industry, Lux is defined as how sensitive the camera is under low light conditions. If you look at a specification sheet, the sensitivity is defined by a number, but more specifically, it will be followed by an F-Number.

 

What does all this mean?   

Before we can interpret the lux rating, it would be helpful to define the scientific meaning of lux.

 

What is "Scientific definition of Lux"?


Lux is an origin unit based on lumen. One lux is equal to one lumen per square meter; lumen is the unit of luminous flux, a measure of the power of light perceived by the human eye.

1 lx (lux) = 1 lm/m2 (lumen per square meter)

The difference between lux and lumen is the lux accounts for area over which the luminous flux is spread. 

For simplicity, let's say a flashlight is 100 lumens and is lit up in an area that is one square meter.  The actual illuminance would be 100 lux.  Now, let's take that same flashlight and take it to an area that is spread out over 10 square meters. This will produce a dimmer illuminance of only 10 lux.

 

The chart below will give you an idea of how bright a lux is in practical terms.

 

How is lux determined?

 

Lux is a subjective value and is determined by each manufacturer's method of testing. You may have seen the sensitivity expressed as 0.5lux, 0.5lux@F2.0, 0.5lux/F1.2, or other variations. There is no standard ISO regulation when it comes to measuring lux, so the same camera may rate a different lux by different manufacturers. The reason is because the manufacturers do not use the same parameters when testing lux.

The outcome will depend on the following factors:

 

F Stop: F stop of a lens is the area of the aperture that determines the amount of light allowed to enter the CCD sensor. The lower the F stop number, the more light it is able to absorb.

As seen in this diagram above, the lower the f number, the bigger the aperture is, which means the absorption of light is much greater affecting the lux of the camera. ie. 1 lux / F10 equal to

0.01 lux / F1.0 since it will absorb 10 times more light than F10 lens.

 

Color Temperature:  The characteristic of visible light that is expressed in Kelvin (K). It is important in which Kelvin environment the camera sensitivity is tested under. As seem in the diagram

below, each color temperature has its own wavelength (nm). High color temperature such 5,000 K or more, represents cool blue, which has low wavelength (~400nm). Low color temperature

such 3,000K below, represents warm yellowish red color, which has higher wavelength (~800nm). However, this difference of wavelength will bring critical difference in lux testing. Spectrum of

500nm light source will produce more electrons than 700nm light source.

 

Reflection ratio: Reflection ratio of the object and its background will significantly distort the measurement. An object with 100% reflection rate will generate 100 times more light on target plane than an object with 1% reflection rate. 

 

IRE: Unit used to measure composite video signals. 100 IRE equals to 700mV, this is full video signal which will operate at the best image, contrast and brightness. 50 IRE is the half of it and 10 IRE is 10% or original amplitude or 70mV. Generally acceptable video signals are at 30 IRE as a minimum requirement for a fair quality video image.

Company A specs it's camera at 0.01 lux @ 1.4 (AGC on).

Company B specs it's camera at 0.1 lux @ F1.4 /50 IRE (AGG off).

At first glance, Company A's camera has a more appealing lux value, but when both cameras are compared side by side under the same conditions, Company B's camera will be much more sensitive under low light.

 

Signal to Noise Ratio: The S/N ratio measures how well a camera signal can transmit in low light conditions. The S/N ratio is expressed in decibels (dB).A highly sensitive CCD will pick up a higher noise level, so in practice the sensitivity is reduced somewhat in order to minimize the noise. Noise is also proportional to temperature, so it is inevitable to have a noise free image without adjusting external factors.

 

Regardless of how low of a lux is specified or it's claim to see in virtual darkness, all non-IR cameras require a minimum amount of light to perform under low light conditions. 

References: (5,6,7)
5. CCTV Networking and Digital Technology (2nd Edition) by Vlado Damjanovski
6. Wikipedia.org - Definitions
7. CM-1UTP Camera Master UTP Adaptor - CM1-UTP.ISB

 

About "Infra Red Illumination"

Infrared cameras are a popular choice among installers because of its ease and flexibility to install in almost any environment.  

However why does some IR cameras perform better than others?

Infrared illumination, also known as IR illumination, is used to provide light to scenes that would otherwise be too dark for the standard CCTV camera to pick up. The best way to see in the dark would be to provide sufficient white light, but this is not always possible. The human eye can see wavelengths of light approximately between 380nm to 760nm.  As illustrated in the diagram below, Infrared (IR) used on CCTV cameras cannot be seen with the human eye.

 

 

How does CCTV camera detect Infrared Light?

The image sensor in the Camera is called the CCD, which is made of Silicon and Germanium. As you can see from Diagram below CCD responds differently to the wavelengths of Light. Therefore for CCTV applications, the material used in the CCD has a great bearing on the response to either visible or infrared light.

 

 

This does not mean all CCTV cameras are made to be sensitive to see with IR illumination. Color cameras are not suitable for IR illumination because of the color filter. Monochrome

(Black and White) cameras will work with IR illumination because it dose not have a filter to restrict the infrared light from passing through the CCD. Color cameras with IR are considered day/night cameras and come equipped with an OLPF filter to allow both visible and infrared light to pass through.

 

Does this mean IR sensitive Filter CCTV camera all perform in same manner?

The Answer is No.

 Not all IR cameras are alike.

 

There are important factors to be considered like CCD type and size, lens size and environmental factors. Depending on type of Lens and CCD sensor Size, Ex-view CCD performs far better than the standard Super HAD due to its high light sensitivity. The size of the CCD, whether it is 1/2", 1/3", 1/4" will also directly affect the IR performance. The size of the CCD will determine the power output (Video Signal) level, so performance will change.

The Lens has an F number (Aperture), which restricts the Amount of light that can be passed through the sensor. As you can see from the chart below, the higher the F number, the lower the percentage of light that passes through the lens.

The Environmental factors are important because the camera will only see the light energy reflected from an object. Regardless of how far the IR's are capable of stretching, if there is no object for the infrared to reflect off of, the infrared light is just swallowed up by the vast darkness and no image is produced.

Here are some typical values for reflectance given below.

Matt white test card 89%

Snowy scene 85%

Glass windows and walls 70%

White matt paint on concrete 60%

Unpainted concrete, car park 40%

Red bricks 35%

Open country, trees, grass 20% ( This can be 60%-70% for I/R)

Empty asphalt area 5%

 

This provides a brief explanation of how infrared works and how it interacts with the cameras. There are many issues to consider before purchasing IR cameras. It is not an all purpose camera and the performance has a direct correlation to the type of CCD used, the F-Stop of the lens and the environment in which it is installed.

References: (8,9,10,11)
8. CCTV Networking and Digital Technology (2nd Edition) by Vlado Damjanovski
9. Wikipedia.org - Definitions
10. CCTV Information introduced by Robyn Sones
11. CCTVForum.com

 

Can Cameras Really See in The Dark Without IR


The answer to this question would have been "No" several years back and people would have looked at you like you were clueless. Infrared cameras have been, and still are, very popular choices because of its flexible use during the day and at night. However, there are those instances where infrared is not preferred or cannot be used.   

It is a known fact that black and white cameras are much more sensitive under low light conditions than color. Ex-view technology was developed by Sony to give even greater sensitivity than the earlier Super HAD CCD's. These image sensors not only provide high sensitivity in the near infrared light region, but also achieve a visible light sensitivity increased by +5 to +7 dB. As you can see in the image below, Ex-view CCD can pick up an image in a room with the doors closed, shades down and the lights off. Of course, this is not an absolute dark room because there are lights bleeding from the window shades and light from the computer monitor.

 

Ex-View       Super HAD

 

 

 

What's the next best thing to Exview? 

Introducing Sens Up...

Today, with the aid of digital programming, a camera can produce a well lit image under very low light conditions, more than what the human eye can see.

Commonly known as Sens Up, it has taken on other names as well. The concept is simple: manipulate the DSS. Increasing the digital slow shutter speed enhances the low light sensitivity, as where the standard day/night function may not be able to pick up an image. The shutter speed is slowed down to the point where it can retain enough light to produce an image. The Sens Up feature has different levels and can be set at 2X, 4X, 8X, 24X, 64X and 128X. (See the comparison pictures up to 64X below)

The higher the setting, the brighter the image will get. However, there is a downfall to this feature.

The brighter the sensitivity, the lower the frame rate, causing the movement to stutter. So there is a trade off: compromise quality to be able to see in the dark? The video is still fairly usable up to 8X. Sens Up is a great for applications where light is not readily available and IR is not an option.

 

References: (12)
12. CCTV Networking and Digital Technology (2nd Edition) by Vlado Damjanovski

 

 

 

Why Sony Super HAD CCD II? 

 The new Sony Super HAD CCD II is a Charge-Coupled Device (CCD), so called an image sensor, mainly designed for security camera applications and provide improved characteristics compared to the existing Sony Super HAD CCD I. This new image sensor provides significant improvements in the sensitivity characteristics, which are critical for security camera applications, while maintaining the same saturation signal level and smear level characteristics as the old Sony Super HAD CCD. 

 

High Sensitivity and New Spectral Sensitivity Characteristics

 The new Sony Super HAD CCD II provides a significantly improved focusing of light onto the photodiodes due to an increased aperture ratio and an improved upper section structure. This suppresses the reduction in collection ratio due to the increased angles of incident light rays that occurs when the lens is used at its widest aperture. Also, by adopting new complementary color pigments in the color filters, Sony increased the sensitivity to the blue end of the spectrum (shorter wavelengths) and achieved well-balanced spectral sensitivity characteristics. 

 

Spectral Sensitivity Characteristics Comparison

The combination of these technological improvements results in a significant increase in the sensitivity characteristics that totals +7 dB. When one compares actual images, one can clearly see that the image quality under the low-light conditions that are critical for security camera applications has been improved significantly. (See photograph below.)


Support for Reduced Amplitude Drive Voltage

The new Sony Super HAD CCD II supports not only the same 5.0 V (typical) level used in the old Sony Super HAD CCD but also a 3.3 V (typical) level for both the horizontal transfer clock voltage and the reset gate clock voltage. This can have a large effect in reducing both power consumption and heat generation. 

References: (13)
13. Sourced from Sony CCD Datasheet