Angle of View & Image Resolution

Transcription

1 White Paper HD Cameras 4500/4900 Series Angle of View & Image Resolution English Rev /

2 1 Abstract Dallmeier HD cameras of the 4500 / 4900 series provide high-quality images at resolutions of up to 3 Megapixel ( ). 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. 2 Validity This document applies to the following Dallmeier HD cameras: DF4500HD DF4900HD DDF4500HDV DDF4900HDV The descriptions in this document are based on the software version 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 By default, the following lens is attached to the cameras DDF4500HDV and DDF4900HDV or is optionally available for the cameras DF4500HD and DF4900HD: Lens Specifications Format 1/2" Type Megapixel varifocal lens Focal Length 4 10 mm Iris Range F Max. Angle of View Miscellaneous DC auto iris Table 2: Lens Specifications 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: pixels / pixels (D1 NTSC / D1 PAL) pixels (720p) pixels (1080p) pixels (2 MP) pixels (3 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: (Sensor: 5.70 mm 4.28 mm, 7.13mm diagonal) (3 MP) (720p) (2 MP) (1080p) (D1) Fig. 1: Used Sensor Areas at Different Image Resolutions By default, the resolution of the output image is the same as the used sensor area. However, due to specific subsampling methods (skipping and/or binning), the used sensor area or input resolution for D1 and 720p is not, as may be expected, in accordance with the output resolution, but considerably differs from it. 3

4 3.4 Angles of View The following table lists the available angles of view at the particular image resolutions: Angle of View (H V) Focal length Resolution * ** f = 4 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 on page 5. 4 Calculations Assuming that the lens is focused to infinity, the angle of view α can be calculated 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. Example: For the calculation of the horizontal angle of view α H at a focal length f of 4 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: 4

5 5 Subsampling 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 and 720p, special subsampling methods are used to reduce the output resolution without reducing the available angle of view. 5.1 Skipping 5.2 Binning 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. 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). 5.3 Pixel Readout 720p (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. 2: Pixel Readout Method at 720p 5

6 D1 NTSC (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. 6

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8 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)