Angle of View & Image Resolution

Transcription

1 White Paper HD Cameras Angle of View & Image Resolution Box Cameras DF4510HD DF4910HD DF4910HD-DN DF4920HD-DN Dome Cameras DDF4510HDV DDF4910HDV DDF4910HDV-DN DDF4820HDV-DN DDF4920HDV-DN IR Cameras DF4910HD-DN/IR DF4920HD-DN/IR English Rev /

2 1 Validity This document applies to the following Dallmeier HD cameras: Box Cameras DF4510HD DF4910HD DF4910HD-DN DF4920HD-DN Dome Cameras DDF4510HDV DDF4910HDV DDF4910HDV-DN DDF4820HDV-DN DDF4920HDV-DN IR Cameras DF4910HD-DN/IR DF4920HD-DN/IR The descriptions in this document are based on the software version Abstract The above-mentioned Dallmeier HD cameras provide high-quality images at all resolutions. However, it has to be considered that the angle of view at a given focal length varies depending on the set image resolution and therefore the used sensor area. This document specifies how the available angle of view is related to the used sensor area and the focal length of a lens. 3 Specifications 3.1 Sensor The following sensor specifications apply to the above-mentioned cameras: Sensor Specifications Format 1/2.5" (4:3) Active Sensor Size 5.70 mm (H) 4.28 mm (V), 7.13 mm diagonal Active Pixels 2592 (H) 1944 (V) Pixel Size 2.2 μm 2.2 μm Colour Filter Array RGB Bayer pattern Table 1: Sensor Specifications 3.2 Lens For the calculations described below, the following lens specifications are used: Lens Specifications Format 1/2" Type 3 megapixel varifocal lens Focal Length mm Iris Range F Max. Angle of View Miscellaneous DC auto iris Table 2: Lens Specifications Note that the actual lens specifications depend on the used camera model or lens type. 2

3 NOTE: The lens format must never be smaller than the sensor format as this would result in image vignetting. In fact, the lens format should preferably be larger than the sensor format, since the imaging quality of a lens may decrease from the centre of its optical axis to the outer edges (e.g. by light fall-off). The angle of view stated in the lens specifications usually defines the maximum angle of view (or more appropriately the angle of coverage ), which the lens is able to reproduce with acceptable quality if the focus is set to infinity, and is not dependant on the sensor size. The actual angle of view of a camera, however, also depends on the used sensor area. 3.3 Image Resolutions Depending on the used camera type, the following image resolutions can be set at Encoder 1: pixels (4CIF) / pixels (4SIF) pixels (D1 PAL) / pixels (D1 NTSC) pixels (720p) pixels (1080p) pixels (2 MP) pixels (3 MP) pixels (4 MP) pixels (5 MP) Depending on the set image resolution, a specific area of the entire sensor is used for image capturing (by on-chip windowing ) as illustrated in the following figure: (4 MP) (3 MP) (720p) (2 MP) Active Pixels: (Active Sensor Size: 5.70 mm (H) 4.28 mm (V); d = 7.13 mm) (1080p) (D1 NTSC) (5 MP) Fig. 1: Used Sensor Areas at Different Image Resolutions 3

4 By default, the resolution of the output image is the same as the used sensor area. However, due to downsampling or specific subsampling methods (skipping and/or binning), the used sensor area or input resolution at 4CIF, 4SIF, D1 PAL, D1 NTSC and 720p is not, as may be expected, in accordance with the output resolution, but considerably differs from it. 3.4 Angles of View The following tables list the available (calculated) angles of view at the particular image resolutions: Angle of View (H V) Focal Length Pixel Resolution * ** f = 3.9 mm f = 10 mm Table 3: Angles of View at Particular Image Resolutions * By Column Skip 3 (horizontal skipping), Row Bin 3 (vertical binning) ** By Column Bin 2 (horizontal binning), Row Bin 2 (vertical binning) For more information about skipping and binning, see section Subsampling ( Wide Angle Function) on page 5. Angle of View (H V) Focal Length Pixel Resolution f = 3.9 mm f = 10 mm Table 4: Angles of View at Particular Image Resolutions 4 Calculations Assuming that the lens is focused to infinity and the exit angle of the image matches the entry angle, the angle of view α can be calculated by using the following formula: arctan denotes the inverse tangent function tan 1. d represents the dimension of the used sensor area in mm (vertically, horizontally or diagonally). For example, to calculate the horizontal angle of view α H, the horizontal dimension (width) d H of the used sensor area has to be used. f represents the stated focal length of the lens. 4

5 Example: For the calculation of the horizontal angle of view α H at a focal length f of 3.9 mm, a horizontal resolution of 1920 pixels and a pixel width of 2.2 μm (as stated in the sensor specifications), the following equations were used: Note that the actual angle of view may differ from the calculated value due to the design of the used lens (e.g. by lens distortion). 5 Downsampling Downsampling is applied by default for the resolutions 4CIF, 4SIF, D1 PAL and D1 NTSC. 6 Subsampling ( Wide Angle Function) By default, the output resolution is the full width and height of the used sensor area as illustrated in Fig. 1. For the resolutions D1 PAL, D1 NTSC and 720p, special subsampling methods can be used to reduce the output resolution without reducing the available angle of view. Subsampling can be activated or deactivated in the camera encoder settings by using the Wide Angle checkbox. 6.1 Skipping Skipping reduces the output resolution by only using selected pixels from the used sensor area in the output image. Entire rows and columns are not read out. For example, the 2 Skipping mode skips one Bayer pair of pixels each for every pair output. The 3 Skipping mode each skips two pairs for every output pixel pair. Note that rows and columns are always read out in pairs. 6.2 Binning Binning works in conjunction with skipping and reduces the output resolution by combining and averaging two or three adjacent rows and columns (charge of adjacent pixels of the same colour). Binning can reduce aliasing artifacts which may occur by skipping, improves low-light performance and increases the signal-to-noise ratio (SNR). 5

6 7 Pixel Readout Method and Output Resolution 4CIF (Downsampling) Used sensor pixels: Output resolution: SIF (Downsampling) Used sensor pixels: Output resolution: D1 PAL (Downsampling) Used sensor pixels: Output resolution: D1 PAL (Subsampling On: Column Skip 3, Row Bin 3 ) Columns/rows shown in grey are skipped/binned, columns/rows shown in white are read out. Used sensor pixels: Read out pixels: Column readout direction Row readout direction Fig. 2: Pixel Readout Method at D1 PAL NOTE: Due to technical reasons, only 3 vertical binning is supported. Instead of 3 horizontal binning, 3 horizontal skipping is used. 6

7 D1 NTSC (Downsampling) Used sensor pixels: Output resolution: D1 NTSC (Subsampling On: Column Skip 3, Row Bin 3 ) Columns/rows shown in grey are skipped/binned, columns/rows shown in white are read out. Used sensor pixels: Read out pixels: Column readout direction Row readout direction Fig. 3: Pixel Readout Method at D1 NTSC NOTE: Due to technical reasons, only 3 vertical binning is supported. Instead of 3 horizontal binning, 3 horizontal skipping is used. 7

8 720p (Subsampling On: Column Bin 2, Row Bin 2 ) Columns/rows shown in grey are binned, columns/rows shown in white are read out. Used sensor pixels: Read out pixels: Column readout direction R G R G R G R G Row readout direction G B G B G B G B R G R G R G R G G B G B G B G B R G R G R G R G G B G B G B G B R G R G R G R G G B G B G B G B Fig. 4: Pixel Readout Method at 720p 8

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12 Specifications subject to change without notice. Errors and misprints excepted Dallmeier electronic Dallmeier electronic GmbH & Co.KG Cranachweg Regensburg Germany Tel.: +49 (0) Fax: +49 (0)