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Abstract
Looking for a digital camcorder for use in performing surveillance? Well, technology improvements in CCD and CMOS image sensors as well as the introduction of new high definition display and recording formats have certainly resulted in the introduction of lots of different digital camcorder models. Because of the wide variation in features and price points, it’s more difficult than ever to select the optimal digital camcorder. Certainly, some models will be better than others in consideration of the specific application for which the camcorder is intended and the features that are required. Because there are so many factors involved in making a selection and since new models are frequently introduced, it’s best to provide guidelines useful when evaluating models intended for a specific application. Then, as new models become available, they can be compared to existing models based on the same guidelines.
The goal of this paper is to provide guidelines that should be considered when evaluating camcorders for use in surveillance applications. These applications are those that would be used by military, law enforcement and special investigative personnel for gathering evidence or performing electronic news gathering and usually include viewing at a distance. While these are useful guidelines for understanding important camcorder specs, prior to making your own camcorder purchase decision, we suggest that you perform your own research of available models and, based on these guidelines, select those models that are expected to deliver the best performance and perform your own tests under real-life situations. You may learn that other factors may be important to your specific application and that your final decision is a compromise of those other factors.
For surveillance, optical image quality is of highest importance. Camcorder resolution and sensitivity are the primary factors to consider, with a variety of related factors such as image and pixel fields-of-view, the pixel size, as well as the frame rate, image stabilization, alternative lens options and lens interchangeability, integration mode, digital zoom and display resolution, user interface, video formats, recording media, manual gain, infrared mode, video compression, external audio inputs, viewfinder, robustness, and video compression.
Camcorder Resolution
Video resolution is affected by a variety of different camcorder specifications. It is well known that high definition camcorders deliver higher resolution than standard definition formats. However, it is important to realize that if the camcorder is used to record image sequences that are later to be viewed (rather than using the camcorder just to view the video output in real time), both the optical resolution and the recording format must be considered in determining the ultimate video resolution. Considered separately, the optical resolution depends not only on the detector array size but also the image and pixel field-of-view. HD recording format also affects resolution. Finally, there are other resolution factors that may warrant consideration in order to determine the ultimate camcorder resolution.
Detector Array Size:
It seems that every camcorder uses a slightly different imaging detector. While the HD recording format can be 1280×720, 1440×1080 or 1920×1080, the imaging detectors may vary quite a bit. As shown in the table, for a selection of mid-range camcorders, detector array sizes can vary from single chip cameras having arrays as large as 1920×1080 (Canon HF-10 and Sony HDRSR11) to three-chip blocks having detectors as large as 1440×1080. The physical size of the imaging detector also varies from camcorder to camcorder.
Image and Pixel Field-of-View:
As shown in the accompanying chart (reference at above link) the combination of the physical size of the camcorder’s imaging detector (for example 1/3”) and the focal length of its objective lens (for example 50mm) result in an image field-of-view (FOV) that is defined as the angle of view from which image information is received. There are separate angles corresponding to the horizontal and vertical directions depending on the aspect ratio of the image. For wide screen video formats, the vertical FOV is 9/16 of the horizontal FOV. All camcorders have zoom lenses where the focal length varies from a wide angle position (with the largest angular FOV) to a telephoto position (with the smallest angular FOV). A smaller field-of-view results in a higher image magnification as would be required to view small objects at a distance.
Smaller imaging detectors create narrower angles of view when used on the same focal length lens. For example, a 50 mm lens delivers a horizontal FOV of 5.5° and 3.7° when used with a 1/2” and 1/3” format detector, respectively. Based on the variation of detector sizes and lens focal lengths, the angular field of view for different camcorders varies accordingly. The image FOV can be determined for any camcorder based on a simple formula.
The pixel FOV (also known as the instantaneous field of view, or IFOV) is a measure of the resolution of an imaging system. Though related, image FOV and pixel FOV are distinct and a camcorder having the narrow image FOV does not necessarily have a corresponding narrow pixel FOV. As shown in the table (reference at above link), while the Canon camcorders have the smallest image and pixel FOV values, the Sony HDR-SR11 has a small pixel FOV despite an image FOV that is nearly twice as wide as those for Canon.
A handy FOV calculator can be used to determine the FOV values for certain candidate camcorders. Pixel FOV is also calculated as are detection, identification and recognition range values for certain specified object sizes at a specified distance.
Recording Format:
The difference between the different HD recording formats is beyond the scope of this white paper. However, the impact of recording format on resolution is critical to imaging performance for surveillance applications and is described here.
As shown in the figure (referenced at the first link in article), standard definition camcorders (with a 4:3 aspect ratio) recorded video with 480 horizontal rows per frame giving a resolution of 640 x 480. (This is for NTSC formats. PAL formats recorded with 576 rows per frame giving a resolution of 640×576). Wide screen versions of these standard definition camcorders with a 16:9 aspect ratio were also available and recorded with slightly larger resolution of 720 x 480 (720×576 for PAL versions). Because of the limitations of the recording format, it would not have been useful to have a high resolution CCD/CMOS detector larger than 720×480 pixels in a standard definition NTSC camcorder except under certain circumstances (like zooming in to view central pixels). However, with the introduction of higher resolution recording formats, higher resolution detectors make sense.
Camcorder Sensitivity
High definition refers to video having higher resolution than standard definition and most commonly refers to resolutions based on either 720 rows/frame or 1080 rows/frame. Recording formats can either be 1920×1080, 1440×1080 and 1280×720. Consequently, are available with either 720 or 1080 rows per frame recording formats having a resolution of 1280×720 and 1920×1080 respectively. It is important to understand the recording format for any camcorder under consideration. If the full sensor resolution is necessary to perform the surveillance function, then make sure that the camcorder records in sufficient resolution.
Had this whitepaper been about digital still cameras, the above discussion of recording format would’ve been sufficient to describe the effects on image quality. However, since video incorporates motion, the fact that some camcorders record “interlaced” video while others record “progressive” video has some relevance, particularly when motion is important to the type of surveillance being performed. If not, then the relevance of interlace vs. progressive scan is less important.
Other Resolution Factors:
It’s worth mentioning that in those camcorders that utilize three sensors, pixel shift technology has been employed in the horizontal axis to make incremental improvements to the sensor resolution by using interpolation. This complicates the calculation of optical resolution somewhat, but may not be a primary factor.
Camcorder low-light sensitivity is usually of primary importance for surveillance applications, particularly when imaging at dusk or dawn, but also when attaching a night vision module that intensifies ambient light. The guidelines for camcorder evaluation are listed below. We recommend a review of the following factors: the Recommended Minimum Illumination Specification, Objective Lens F-number, Detector Pixel Size, Night Vision Module availability.
As such, but criteria enabling the comparison of these including: objective lens F-number, pixel size, electronic noise reduction, It is apparent that low-light performance as detailed on the list of the camcorders specifications because different cameras can produce varying results, even though the specifications may be similar.
Consequently, once you’ve identified a few different camcorders that have the features of interest and the performance that you require, it is important to test the camcorder’s low-light capabilities.
Recommended Minimum Illumination Specification:
Quite often, low-light performance specifications are provided on a camcorder’s specification sheet. These are usually described as either “low light sensitivity” or “minimum illumination” and the result is given in lux. The lower the lux rating, the better the camcorder performs in low light. However, because the details describing how these low-light measurements are made is not often provided, it is extremely difficult to compare these figures when comparing different camcorder models from different manufacturers. (What’s the assumed reflectivity of the objects being viewed? What’s the signal-to-noise ratio? Image brightness? etc.). As such, use of these specifications is complicated and not often satisfying. It is obviously best to test the camcorders yourself. Some websites (such as www.camcorderinfo.com) perform their own very rigorous tests.
Detector Pixel Size:
Not only is the detector array size important, but the physical size of the detector is also important for a number of reasons. Most often, small detector physical sizes (such as 1/6” format) are found in less expensive camcorders while larger detectors (e.g. 1/3” format) are found in higher performance camcorders. Physically small detectors require small, less expensive lenses and result in compact, lower cost camcorders. For surveillance applications, an important consideration is that light sensitivity decreases with pixel size. While larger pixels in general have more light sensitivity and are superior when imaging in low light or at night, larger pixels also result in a decrease in resolution (in consideration of a particular focal length lens). Also, larger pixels with large array sizes result in large arrays and large lenses. So, smaller pixels are often preferable. One should compare the sensitivity of camcorders with the smallest pixels with other camcorders being considered. Pixel sizes for a variety of digital camcorders are shown in the accompanying chart (referenced at the first link in the article).
Objective Lens F-Number:
As with SLR cameras, lens F-number is extremely important to a camcorder’s low light sensitivity. All other things being equal, a lens with a low f-number (such as F1.4) focuses much of the light from the scene onto the camcorder’s sensitive imaging detector. A lens with an F-number that is twice as high (F2.8) will pass only 25% of the light through it, making the camcorder 25% as sensitive. That’s a lot! For compact camcorders with integral lenses, sometimes the objective lens must be designed with a relatively high F-number in order to retain the compact design yet deliver the desirable zoom range. In particular, one finds that the F-number at the longer focal length values increases slightly. F-numbers at the maximum zoom setting for a variety of digital camcorders are shown in the accompanying chart.
Night Vision Module Availability:
For camcorders to be used for surveillance applications, the availability of a night vision module can be quite important, particularly for imaging at a distance and at night. Camcorders simply do not have sufficient light sensitivity to enable imaging in these environments. However, by attaching a night vision module such as those shown here, the light sensitivity of the camcorder can be increased by up to ten F-stops. As is well known, the resulting night vision images will be green, absent of all color. But, reasonable images can be produced in starlight and moonlight. Some of these night vision modules can be seen by clicking on the link above.
Other Factors To Consider
In evaluating camcorders for surveillance applications, resolution and sensitivity are usually the primary considerations, other factors may deserve attention. Some of these are listed below.
Media: “Media” refers to where the video will be stored. Many camcorders utilize several different media types. The following are the common types of media and some considerations for surveillance applications:
MiniDV Tape – These have been used for many years. The downside is the need to fast forward and rewind rather than direct access. However, they are known to be quite reliable, they are widely available and rugged. In addition, it is often convenient to have physical media with surveillance footage rather than a soft copy only which could more easily be misplaced or mislabeled.
MiniDVD – Though a familiar media, it may not be as convenient as one expects since the DVD often cannot be directly inserted into DVD players. Also, recording in the highest quality sometimes results in significantly lower capacity.
Hard Disk Drive (HDD) – These are generally high capacity media (e.g. 120 gigabytes) with long record times. Backup is obviously important and necessary. Also note carefully if compressed video is being used for storage in order to increase the available capacity.
Flash Memory – Flash memory (such as solid state memory and memory cards) is very attractive since there are no moving parts resulting in a compact and efficient design. Capacities are increasing every year.
Image stabilization:
Image stabilization is a camcorder feature that helps improve image quality during hand-held video recordings, reducing blur due to camcorder motion. The stabilizer instantly corrects camcorder shake so that hand held shots at full telephoto (such as video taken from a moving car) are smooth and steady. These image stabilizers can be either optical or digital in design. Optical image stabilizers usually manipulate a prism in the lens to stabilize the image when projected on the camcorder detector. Digital image stabilizers usually sense the camcorder motion and, taking advantage of extra rows and columns on the perimeter of the detector, shift the image an appropriate amount to stabilize certain types of motion. Normally, the optical image stabilizers, though more expensive, are preferable since they better remove the blur component of the motion.
Lens interchangeability:
Many lower cost camcorders have non-removable objective lenses. These lenses are compact and highly integrated into the camcorder’s housing, have a very wide zoom range and even macro capability. Normally, the field-of-view of the integrated zoom lens is selected to achieve the FOV range desired for most applications. However, for increased versatility, higher-end camcorders offer a removable lens and a lens mount so that a series of lenses can be offered for a wider variation of applications, including wider angle and more telephoto viewing. (See a list of mid-range camcorders having removable lenses here). For non-removable lens camcorders, if wider angle or more telephoto FOV values are required, adapters are sometimes offered to change the magnification as desired. But the result is a reduction in optical quality. For the surveillance videographer, interchangeable lenses can be a great benefit if the magnification offered by fixed lens camcorders isn’t sufficient for the desired application. In addition, night vision modules that are designed for removable lens camcorders fit between the lens and camcorder and retain all that objective lens’ functions including motorized zoom, auto-iris and image stabilization.
Digital zoom:
Digital zoom is an important feature for any camcorder. Because it is not possible to appreciate all the imaging detector’s resolution on the camcorder’s viewfinder or display panel, digital zoom enables the operator to view details on the viewfinder that would otherwise go unnoticed until digital images were later analyzed. In addition, it’s useful to note that a detector having a larger pixel array enables the operator to perform more digital zoom than with a camcorder having a smaller detector without the effects of pixelization.
User interface:
Any camcorder operator will confirm that accessibility to features is of some importance. Drilling down through sub-menu after sub-menu to access routine functions is not a desirable thing to do while in the field.
Infrared mode:
Some camcorders feature a sensitivity to near infrared light permitting the use of an infrared illuminator to brighten scenes. For a variety of reasons, these illuminators are not applicable for many surveillance situations. For one thing, normally the illuminators included with the camcorders do not project sufficient illumination at the distances of interest. In addition, they can be detected by others with most camcorders and CCD cameras.
Manual Gain:
For low-light situations, some camcorders offer a manual gain mode that permits the user to manually adjust the gain in the video image, usually increases up to +18dB. Though grainy, for surveillance applications, the video images are normally a significant improvement over non-gained or even auto-gained versions.
Shutter Speed:
If maintaining the standard frame rate (for example 60Hz) is not necessary because the objects under observation are not moving quickly, the frame rate and detector shutter speed can be reduced (for example to 1/3 sec) so that each of the sensor’s pixels will integrate light over a longer time that they are exposed. In darkened environments, selecting the longest acceptable integration time may be sufficient to provide a usable image at dusk or dawn.
External Audio inputs:
In order to prevent the appearance of sounding like a pure optics geek, it is important to mention that audio quality and access to external audio inputs is important for the surveillance videographer. The built-in microphone doesn’t always work well when shooting images far away or when you need something more directional to filter out ambient noise.
Viewfinder:
When performing covert surveillance, having a viewfinder on the camcorder is sometimes important in order to prevent the illumination from the bright LCD panel.
Robustness:
Another serious compromise is the lack of robustness in camera hardware — the flimsy switches and controls that cause videographers stress. When you’re out in the field under possibly dangerous conditions, you don’t want to be playing around with little controls and LCD panels that easily come apart.
Price:
Of course, price is always a consideration when evaluating the many many camcorders models available today. But, what these guidelines should show is that very often, the most expensive camcorder may not be the best for a specifi c application. Understand what features are most beneficial for your application in consideration of camcorder price range.
Conclusion
Technology improvements in CCD and CMOS image sensors as well as the introduction of new high definition display and recording formats have resulted in the introduction of lots of different digital camcorder models. Guidelines have been provided when evaluating camcorders for use in surveillance applications.
For surveillance, optical image quality is of highest importance. Camcorder resolution and sensitivity are the primary factors to consider, with a variety of related factors such as image and pixel fields-of-view, the pixel size, as well as the frame rate, image stabilization, alternative lens options and lens interchangeability, integration mode, digital zoom and display resolution, user interface, video formats, recording media, manual gain, infrared mode, video compression, external audio inputs, viewfinder, robustness, and video compression.
While these are useful guidelines for understanding important camcorder specs, prior to making your own camcorder purchase decision, we suggest that you perform your own research of available models and, based on these guidelines, select those models that are expected to deliver the best performance and perform your own tests under real-life situations. You may learn that other factors may be important to your specific application and that your final decision is a compromise of those other factors.
DISCLAIMER: We have made every attempt to provide accurate information. However, we cannot accept any responsibility for errors or inaccuracies. Should you identify errors or require assistance, please contact us directly. Thank you.
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