Transcription
White paperLighting for Network VideoLighting Design Guide
Table of contents1. Introduction32. What is light?33. What is color?34. What is infrared light?45. Color or monochrome images?46. Brightness and glare47. Light and surfaces48. Light sources69. Network video lighting - which wavelength?710. Light and safety711. Beam patterns812. The inverse square law913. Illumination distances for Axis products914. Using multiple illuminators1015. Measuring light1016. The need for even illumination1117. Specifying the correct camera1118. Specifying the correct lens12
1.IntroductionWhen selecting a network camera for day or night surveillance, there are several elements impactingimage quality that are important to understand. This guide is intended to give a basic overview of thoseelements, to give an understanding of how lighting affects the image, and of the factors that need to betaken into consideration for creating favorable lighting in dark environments.2.What is light?Light is fundamental to network video. It is light reflected from the scene being viewed that allows images to be visible both to the human eye and to the camera. So the performance of any network videosystem depends not only on the camera and lens, but also on the quantity, quality, and distribution ofavailable light.Light is energy in the form of electromagnetic radiation. The light’s wavelength (or frequency) determines the color and type of light. Only a very narrow range of wavelengths is visible to the human eye,i.e. from approximately 400nm (violet) to 700nm (red). However, network video cameras can detect lightoutside the range of the human eye, allowing them to be used not only with white light, but also withNear Infrared light (715-950nm) for night surveillance.The behaviour of light varies according to the material or surface it strikes, where it is either reflected,diffused, absorbed or (more commonly) subjected to a mixture of these effects. Most surfaces reflectsome element of light. Generally, the paler the surface, the more light it reflects. Black surfaces absorbvisible light, while white surfaces reflect almost all visible light. Infrared is not always reflected in thesame way as visible light. The way infrared is reflected depends on the nature of the material – see thereflectance chart on page 7.3.What is color?The processes by which the human eye and brain “see” color are very complex, so the definition of colorpresented here is of necessity greatly simplified.Light at wavelengths visible to the human eye is interpreted by the brain as colors; from 400nm (violet)to 700nm (red). All other visible colors are between these wavelengths - indigo, blue, cyan, green, yellowand orange, as seen in e.g. a rainbow. When viewed together, these wavelengths appear as white light.A green leaf looks green because it reflects green wavelengths present in white light. If you look at itunder a red light it will appear black, because the lighting contains no green. The same applies when youbuy a colored item of clothing - you might take it to the door or window to check how it looks in daylight.This is because interior lighting contains a slightly different mixture of wavelengths from the light outside, and consequently alters the apparent color of the garment.The exact same can be said for network video. The color output of an illuminator affects the color seenby the camera, as in e.g. the yellowish light seen under sodium street lighting. To provide true color network video images, white light illuminators need to provide color-corrected illumination matched to thevisible spectrum.Colored objects reflect light selectively. They reflect only the wavelengths (i.e. colors) that you see, andabsorb the rest. A red flower, for instance, contains pigment molecules that absorb all the wavelengthsin white light other than red, so that red is the only color it reflects.At wavelengths lower than the visible spectrum, the radiation becomes ultraviolet (UV), which burns theskin (tanning) and is therefore unsafe for network video. At wavelengths higher than the visible spectrum, the radiation becomes infrared (IR).3
WAVELENGTH IN METERS10110 -110 -210 -310 -4RadioTV10 -510 -610 -710 -810 -910 -10 10 -11 10 -12 10 -13Infra-RedX-RayMicrowaves10 810 910 1010 11Ultraviolet10 1210 1310 1410 1510 1610 17Gamma Rays10 1810 1910 2010 21FREQUENCIES IN HZWAVELENGTH IN UE400VIOLETFigure 1: The wavelengths of visible spectrum4.What is infrared light?Infrared light (IR) is light at higher wavelengths that the human eye can see. The infrared light used fornetwork video lighting is at wavelengths slightly greater than the visible spectrum, i.e. between 700 and1100nm. This IR range is also known as Near Infrared (NIR) light.As the camera can “see” some infrared light that is invisible to the human eye, there are various alternatives as to how this can be displayed on a computer screen. Usually the image is shown in black andwhite, with the scene appearing as it would if the human eye could see infrared light. Other false colorscan also be used to show the content of infrared light compared to visible light. This is sometimes usedin scientific imaging.Applications that require covert surveillance, or which must otherwise avoid even low levels of visiblelighting are ideal for infrared light.5.Color or monochrome images?The first question to answer when setting up night time surveillance lighting is whether to go for coloror monochrome images. Color is preferable in many cases, but care must be taken to provide true color,which can be achieved by using a color-corrected illuminator. Consider the yellow light provided by lowpressure sodium street lighting - using incorrect white light can actually impair performance and lead toinaccurate color rendition – a camera is only as good as the available light.Infrared should be the method of illumination in all cases where white light would be too intrusive orwhere covert surveillance is required. Infrared lighting can also illuminate at greater distances thanwhite light at the same power level.6.Brightness and glareBrightness is the subjective perception of luminance from a given area. Glare is the result of excessivecontrast between bright and dark areas within the field of view. This problem is greater in darkness, whencontrast between bright and dark areas make it difficult for the human eye (and network video camerasusing infrared) to adjust to changes in brightness.7.Light and surfacesLight at wavelengths visible to the human eye is interpreted by the brain as colors; from 400nm (violet)to 700nm (red). All other visible colors are between these wavelengths - indigo, blue, cyan, green, yellowand orange, as seen in e.g. a rainbow. When viewed together, these wavelengths appear as white light.4
7.1.DiffusionA diffusing material scatters light passing through it. The direction and type of light changes as itpasses through the material.Diffused lightscattered inall directionsNormalAngle ofIncidenceFigure 2: Light diffusion7.2.ReflectionWhen light hits a surface it can bounce back as a reflection. The quality of the surface affects the typeof reflection. Highly textured surfaces scatter light, due to tiny irregularities in the material, whilst a flatsurface such as a mirror provides a more focused reflection.7.2.1.Specular reflectanceIf a surface reflects light like a mirror - it is said to have specular reflectance. With specular surfaces,the angle of incidence is equal to the angle of reflectance.Angle ofReflectionNormalAngle ofIncidenceFigure 3: Specular reflection7.2.2.Diffuse reflectanceDiffuse reflection surfaces bounce light in all directions due to tiny irregularities in the reflective surface.For example, a grained surface will bounce light in different directions. A diffuse reflective surface canscatter light in all directions in equal proportions.Angle ofReflectionNormalDiffuseReflectiveSurfaceAngle ofIncidenceFigure 4: Diffuse reflection5
7.2.3.Retro-ReflectionIn this type of reflection the surfaces bounce light back in the direction it came from. Traffic signs andvehicle number plates have retro- reflective surfaces.NormalRetro- ReflectiveSurfaceAngle ofIncidence Angle ofReflectionFigure 5: Retro-reflection7.3.Reflectance levelsReflectivity is a measure of the reflected power compared to incident power. Objects reflect light at different intensities, and energy not reflected is absorbed and converted to heat. Objects with low reflectivityabsorb a lot of energy – which is why e.g. a brick wall feels warm in sunlight. See below.It is important to remember that the camera does not use the ambient light on a scene as detected by alight meter, but instead the amount of light reflected by objects in the scene.7.4.AbsorptionSome surfaces actually absorb light. Colored surfaces absorb some light and reflect the rest – which is whythey appear a particular color. A black surface absorbs most of the light falling on it. The light energy isusually turned into heat, so dark materials heat up easily.NormalAngle ofIncidenceFigure 6: Light absorption8.Light sources8.1.Incandescent lamps (including halogen)Incandescent bulbs were the first bulbs developed and are highly inefficient, wasting 90% of input energy as heat, making them hot to touch. Halogen bulbs provide a minimal increase in efficiency but stillwaste up to 85% of the input energy as heat. For network video purposes, incandescent bulb life is limited and they are very inefficient.8.2.Fluorescent lampsUse of these lamps for network video purposes is limited, due to the “pulse” effect perceived when thescene is viewed with a network video camera. These lamps are generally low power and designed mainly for internal fitting. As they have a large diffused source, the light output is difficult to focus andcontrol.6
8.3.HID (High Intensity Discharge) lampsThese are efficient, provide good color rendition and have a long life – up to 12,000 hours. HID lampscould well be used in network video, but they suffer from long start-up times (2-3mins) and cannot beturned on again immediately after being turned off.8.4.LEDsLight Emitting Diodes are the fastest growing lighting solution for network video applications. Theirefficiency is typically 80-90%, with the greatest efficiency coming from LEDs producing red light. TheLEDs are often chosen in network video applications because of their advantages, which includeextremely low electrical consumption; low operating temperatures and continuity of color throughoutthe unit’s operating life.Unlike traditional bulbs, LEDs are also highly durable, insensitive to vibration, and their hard casingmakes them difficult to break. They are also capable of emitting light at a given wavelength without theneed for a filter and are quick start devices.LEDs offer the lowest possible running costs (less than 100 watts for the highest power units) with thelongest operating life - up to 100,000 hours (10 years).In comparison, fluorescent bulbs typically last 10 000 hours and incandescent bulbs 1000 hours.9.Network video lighting - which wavelength?White light: A mixture of light at 400-700nm provides true white light.Practical uses: Illuminate an area for the network video system Improve the overall level of illumination for personnel Provide a welcoming environment for authorized personnel Deter crime by illuminating a secure area upon intrusion Can be used with monochrome, color, and day/night camerasInfrared: 715-730nm. Overt IR produces a red glow like a red traffic light. 815-850nm. Semi-covert IR with a faint red glow. 940-950nm. Covert IR - invisible to the human eye.Practical uses of infrared: Provides discreet or covert illumination for network video Minimizes light pollution Provides very long distance illumination Can be used with monochrome or day/night cameras10.Light and safetyAs white light is visible to the human eye we have a natural protection against overexposure to whitelight. The iris and the eyelids close to reduce the input of visible light. If this is not enough, we simplyturn away from the light. As we cannot see infrared light, our eyes cannot automatically adjust to overexposure. However, infrared does produce heat – which can be used as a safety measure. If you can feelthe heat of the IR unit - do not look at the source. Even the most powerful IR units, at angles of 10degrees, are eye safe beyond distances of 2 meters.7
11.Beam patternsTo provide illumination for network video, the angle of illumination should be adjusted so as to light thewhole scene adequately. Modern Adaptive Illumination units allow the angle of illumination to be adjusted onsite, to suit the specific scene requirements. Lighting that is too narrow will produce “whiteout” or glare in the middle of the scene, with some areas not correctly illuminated.LightCameraFigure 7: Lighting is too narrow for camera field of viewLighting that is too wide means “wasted” light and reduced viewing distance.CameraLightFigure 8: Lighting is too wide for camera field of viewMany installations use varifocal lenses, and ideally there should be the same level of flexibility as regardslighting, so as to maximize system performance. Flexible illuminators for video surveillance, such asthose in Axis’ portfolio, offer a range of output angles - allowing you to select the angle that covers theexact field of view and provides the best images. Adjustment is quick and convenient, and the availableangles are easily selectable.widenarrownarrowAdaptive Illumination angle settingwideFigure 9: Adaptive illumination to cover many angles of view8
12.The inverse square lawThe amount of light available at a particular distance is inversely proportional to the square of the distance from the light source. As light obeys this inverse square law, we’ll now look at how this law isapplied.As light travels away from the point source it spreads both horizontally and vertically, with less light atgreater distance. In practice, this means that if an object is moved from a given point to another pointtwice as far from the light source, it will then receive only ¼ of the light (2 x distance2 4).Taking this further, if an object 10m from a light source receives 100 Lux, moving the object to 40m fromthe source means it will receive only 1/16th of the light (4 x distance2 16) resulting in the object receiving only 6.25 Lux. The inverse-square law applies to both white light and infrared light in the same way.Figure 10: The inverse square lawIllumination distances for Axis productsThe chart shown below is a guide to selecting an appropriate Axis infrared illuminator for the camera’shorizontal angle of view and the distance to the object from the illuminator. Note that the solid areadenotes optimum usage. The shaded area denotes less-than-optimum usage.Approximate maximum illumination distances - per camera angle and per illuminator model (distances not to scale)Approximate maximum illumination distances - per camera angle and per model (distances not to m165mT90A33T90A3320oHorizontal angle of veiw 40160oAxis IR Illuminator modelsAXIS T90A01 50o (fixed)AXIS T90A11 50o (fixed)AXIS T90A20 120-180oAXIS T90A21 50-100oAXIS T90A32 30-60oAXIS T90A33 10-20oAXIS T90A40 120-180oAXIS T90A42 30-60o170o180oSolid area denotes optimum usage. Shaded area denotes less-than-optimum usage.Figure 11: IR Illuminator selection chart9
14.Using multiple illuminatorsThe inverse-square law explains how the amount of light drops over distance, but it can also be used tocalculate how many additional illuminators are needed to achieve specific increases in distance.If the distance from a single illuminator is doubled - then the amount of the light is reduced to 25%. Soto illuminate at twice the distance possible with one illuminator (keeping the same power at the scene)then 4 illuminators will be needed (22 4). Similarly, to achieve 3 times the distance of one illuminator,9 illuminators will be required (32 9).The inverse square law can also be used to calculate the effect of using multiple illuminators, by takingthe square root of the change in available light at source. For example, using 4 illuminators will producea 2-fold increase in distance ( 4 2), and using 25 illuminators will result in a 5 fold increase in distance( 25 5).TIP 1! It is not always necessary to use multiple illuminators to achieve increases in distance. Illuminatorswith narrower angles or more powerful illuminators may both provide the required additional increase indistance.TIP 2! If you only need to illuminate a particular object at a particular distance, using e.g. a zoom lens, thenthere is also the option to place a small illuminator close to the object. An example of an application of thistype is a gate or door at a site perimeter, relatively far from the site’s buildings and other infrastructure.1 UNIT 1 x DISTANCENo. .22.42.62.834 UNITS 2 x DISTANCEFigure 12: Units required to cover 2 and 3 times the distanceTIP 3! Doubling the illumination distance requires 4 x the power.TIP 4! Doubling the number of illuminators provides a 1.4 x increase in distance.15.15.1.Measuring lightWhite lightWhite light is measured in lux, the SI unit of illuminance, which also takes into account the area the lightis spread over. 1 lux 1 lumen per square meter.The foot candle is still widely used as a unit of measurement: 10 lux 1 foot candle.10
White light at the scene can be measured by the simple use of a light meter. Typical lux light levels are:Bright sunny day 10,000 - 100,000 luxStreet lighting 5 luxOvercast day 1,000 - 10,000 luxFull moon 0.1 luxTwilight 1 - 100 luxBright, clear starlight 0.01 - 0.0001 luxFigure 13: Light intensity for various scenarios15.2.16.InfraredAs lux is a measurement of visible light, and by definition infrared produces invisible light, lux cannot beused to measure infrared power. The most common form of measurement for infrared light is mW persquare meter, a simple statement of energy output from a light source over a given area.The need for even illuminationThe most important aspect in designing any lighting system is achieving even illumination. Both the human eye and the network camera/lens need to handle dramatic differences in the amount of light within the field of view.When driving at night on an empty road at night you can see clearly using only the headlights from yourown car. However, when a car approaches from the opposite direction, although the light on scene actually increases, your night vision will still be impaired, as there is now a very strong light around thecentre of the scene causing the iris in your eye to close. The same thing happens with a network videocamera – a bright spot within the image will cause the lens to close and reduce night-time performance.To achieve the best images at night, the illumination must be evenly distributed, using lighting productsdesigned for this purpose.17.17.1.Specifying the correct cameraSensitivityThis describes a camera’s sensitivity to light and essentially measures the minimum light level needed toproduce acceptable images, although this value is very subjective. An image may be acceptable to oneperson and totally unacceptable to another.Sensitivity is typically measured in lux, with camera manufacturers stating the minimum lux level needed to provide acceptable pictures. However, this statement does not usually specify if the minimum luxfigure represents the minimum light on scene, at the lens, or at the camera chip. For Axis cameras, thisvalue always applies to the light on scene.Although lux claims tend to be overstated, and although minimum lux only describes a cameras performance with visible light, this lux value is still the only easily available measure of a camera’s sensitivity.11
TIP 1! There is no such thing as a zero lux camera – every camera needs light to produce high qualityimages. Even the most light-sensitive camera will produce higher signal/lower noise pictures when thereis more light available. The exception is thermal cameras, which create images based on the heat thatradiates from a vehicle or person, allowing the cameras to produce images also in complete darkness.TIP 2! For more information about light sensitivity, you can also refer to Axis’ white paper - In the bestof light - available at www.axis.com/corporate/corp/tech papers.htm.18.18.1.Specifying the correct lensF-stopThe f-stop (aperture) of a lens determines how much light passes through it to the camera chip. In simpleterms, the lower the f-stop, the more light passes through the lens, although the quality and manufacture of the lens also affects the amount of light that can pass. The table below shows the impact ofusing different aperture lenses in a network video system:f/NumberLight passed in %Amount of lightneeded to achieve1 lux at sensorf/1 20%5 luxf/1.215%7.5 luxf/1.4 10%10 luxf/1.67.5%13.3 luxf/1.86.25%16 luxf/2 5%20 luxf/2.43.75%30 luxf/2.8 2.5%40 luxf/4 1.25%80 lux a full f-stopFigure 14: F-stops and light levels required to achieve 1 lux at the sensorTIP! For most camera sensors, the lower the f-Stop of a lens, the more light will pass to the sensor.For a zoom lens, the best f-stop is only achievable at the wide setting. As the lens is zoomed, the aperture closes, which affects how much light is needed on scene to produce good images at low light levels.18.2.TransmissionThe efficiency of a lens is measured by its transmission. Passing through the lens, some of the light willbe lost as a result of the lens material, thickness and coating characteristics. A lens with a higher efficiency will pass a higher percentage of the light. Whilst the f-stop of a lens describes how much light thelens will pass, it is not a measure of its overall efficiency.TIP! The transmission of a lens changes with wavelength. For example, one lens may pass 95% of visiblelight and 80% at 850nm infrared, whilst another may pass 95% of visible light and 50% at 850nminfrared. When specifying the lens, consider the wavelength of light it will be used with. Also note thatglass lenses tend to be more efficient than plastic lenses.12
18.3.Corrected lensesInfrared Corrected lensesInfrared corrected lenses are designed to remove the problem of focus shift between day and night light,using specialist glass and coating technology to minimize light dispersion. Focus shift is caused by thedifferent wavelengths of light. Each individual wavelength focuses at a different point after passingthrough the lens.Color corrected lensesLight sources, including the sun, produce a broad spectrum of lighting. White light as we know it is simply the range of the lighting spectrum visible to humans. As a result, a lens must control which lightpasses through to the camera, so as to create an image accurate to the images as perceived by the human eye. Many cheap lenses do not efficiently match their color passing with the visible spectrum, so asa result they provide inaccurate color images. Color corrected lenses pass only visible light and focuseach individual color at the same point - providing true color and sharp images.Most color-corrected lenses are not suitable for use with infrared lighting, although there are someexceptions.All images in this white paper courtesy of Raytec - www.rayteccctv.com13
41222/EN/R1/1012www.axis.comAbout Axis CommunicationsAxis is an IT company offering network video solutionsfor professional installations. The company is the globalmarket leader in network video, driving the ongoingshift from analog to digital video surveillance. Axisproducts and solutions focus on security surveillanceand remote monitoring, and are based on innovative,open technology platforms.Axis is a Swedish-based company, operating worldwidewith offices in more than 20 countries and cooperatingwith partners in more than 70 countries. Founded in 1984,Axis is listed on the NASDAQ OMX Stockholm under theticker AXIS. For more information about Axis, please visitour website at www.axis.com 2010 Axis Communications AB. AXIS COMMUNICATIONS, AXIS, ETRAX, ARTPEC and VAPIX are registered trademarks or trademark applications of Axis ABin various jurisdictions. All other company names and products are trademarks or registered trademarks of their respective companies. We reserve the rightto introduce modifications without notice.
9. Network video lighting - which wavelength? 7 10. Light and safety 7 11. Beam patterns 8 12. the inverse square law 9 13. illumination distances for axis products 9 14. Using multiple illuminators 10 15. Measuring light 10 16. the need for even illumination 11 17. Specifying the correct camera 11 18. Specifying the correct lens 12
