Spectral and Polarization Configuration Guide for MS Series 3-CCD Cameras Geospatial Systems, Inc (GSI) MS 3100/4100 Series 3-CCD cameras utilize a color-separating prism to split broadband light entering the camera through the lens into three optical channels. An optical trim filter and CCD imaging array are placed at each of the three exit planes of the prism. The image acquired by each array is formed by the wavelengths of light that have been passed through each optical path in the prism. The prism geometry is similar to that used in 3- CCD cameras such as electronics news gathering (ENG) cameras and broadcast quality studio cameras. However, in these cameras the prism assembly is designed specifically for acquisition of the red, green, and blue visible color bands and cannot be modified to support other spectral separations. MS cameras are designed to allow flexibility and fine control over the spectral content of the light impinging on each array. This process and the design issues related to configuring the system to meet various spectral specifications are detailed below. The MS cameras are available in three standardized spectral configurations and one polarization configuration. High quality color imaging is provided with the RGB (red, green, blue) configuration. Color-infrared imaging (CIR) is based on green, red, and near IR bands. The RGB/CIR configuration provides red, green, blue, and near IR bands but resolution in the green and blue bands is somewhat reduced. Finally, the polarization configuration provides imaging of three polarization states with the three image channels. Customized configurations are also available and built to customer specifications. The degree of customization required to achieve a specific spectral separation depends upon where the bands fall, how close together they are, and how narrow the pass band is. RGB CONFIGURATION Figure 1. 3-CCD RGB Configuration We ll begin with a detailed examination of a standard RGB configuration as shown in Figure 1. The color separating prism is actually comprised of three prism components that are bonded 1
together to form a single prism assembly. Broadband light gathered by the lens enters the front face of the prism. Dichroic Coating #1 acts as a long pass filter and reflects all light below 505 nm and transmits the remaining light. The light that was reflected returns toward the entrance face, but reflects off of this face via total internal reflection and exits from the prism. The longer wavelength light that was transmitted through Dichroic Coating #1 passes through the second component of the prism and impinges on Dichroic Coating #2. This coating also acts as a long pass filter, reflecting light below 575 nm and passing the remainder through. The light reflected at Dichroic Coating #2 also returns toward the entrance face of the prism. However, a very thin air gap between the first and second components of the prism causes the second band of light to be reflected via total internal reflection to the exit face of the second prism component. The remaining light that has now passed through Dichroic Coating #1 and #2 continues and exits at the back of the third prism component. In all cases light enters and exits perpendicular to the prism faces, preventing refraction and enabling the dichroic coatings to transmit at peak efficiency. As described above the two dichroic coatings on the prism surfaces determine the spectral composition of the light leaving the prism at each of the three exit planes. However, most applications generally require a narrower spectral band than those created by the successive filtering of the prism. To provide further spectral selectivity, optical trim filters are placed at each exit plane of the prism between the prism and the image sensor. The trim filter passes the passband of interest and provides blockage of any out-of-band light. The use of trim filters is very important to provide accurate spectral control and to insure that out-of-band light does not compromise the image spectral integrity. The spectral transmittance of the standard RGB prism and filter set is shown in the graph below. Transmittance 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 400 450 500 550 600 650 700 Wavelength (nm) Figure 2. RGB Configuration Spectral Transmittance 2
CIR CONFIGURATION By changing the transition wavelengths of the two dichroic coatings on the prism surfaces, another very useful combination can be derived. CIR configuration images green, red, and near IR bands as shown in Figure 3. These bands have been determined to be very useful in assessing the characteristics of plants and organic materials such as produce. CCD imaging sensors are sensitive to wavelengths ranging from about 400 nm to 1100 nm, with peak sensitivity at approximately 500 nm. The tail-off of the CCD s sensitivity at the far end of this range also affects the final spectral content of the image acquired in the IR band. Figure 3. 3-CCD CIR Configuration Multispectral or color-infrared (CIR) imagery is an important data source in the remote sensing industry. Historically, the bulk of CIR imagery has been acquired from sensors onboard earthsensing satellites. GSI s standard CIR configurations are specifically tailored to be compatible with the needs of the remote sensing community. The MS4100 can be configured with commercial narrowband filters. These filters are available in a broad range of bandpasses and center wavelengths. Narrower bands tend to provide a sharper image but reduce the amount of available light. Customized filter selection is also available. The spectral response of our standard CIR configuration is shown in Figure 4 below. 3
Transmittance 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 500 600 700 800 900 Wavelength (nm) Figure 4. Spectral Transmittance of CIR Configuration 4
RGB/CIR CONFIGURATION In order to understand the RGB/CIR 3-CCD camera configuration, we must first examine how a single-chip color camera works. In this type of camera, all of the wavelengths of visible light impinge upon the single color imaging sensor. A mosaic or grid of tiny color filters is placed on the face of the CCD array to filter light so that only red, green or blue (RGB) light reaches any given pixel. The most common layout for the grid of color filters is the Bayer pattern consisting of rows of red-green-red-green. and blue-green-blue-green pixels as shown in Figure 5. A single-chip color camera using an array with n x n pixel resolution is used to generate a threecolor RGB image with n x n pixels for each color plane. However, on the chip only 25% of the pixels are devoted to blue, 25% to red, and 50% to green. (The increased number of green pixels simulates the increased sensitivity of the human eye to green light.) Therefore the resolution of the raw data for each color plane is much less than n x n pixels. These lower resolution color plane images are combined to create a full resolution color composite image through application of a Bayer pattern de-mosaic algorithm. This algorithm derives a red, green, and blue intensity value for each pixel location based on the values of the surrounding pixels. Figure 5. Bayer Mosaic Pattern of Pixel Filters By using a similar process, the MS cameras are able to acquire four spectral bands of imagery from a three-channel optical system. As shown below, our RGB/CIR configuration uses the first dichroic coating to reflect the green and blue wavelengths to the first exit plane of the prism. A color CCD imaging sensor with Bayer pattern filters is placed in this channel. This imaging sensor is used to acquire blue pixels at 25% resolution and green pixels at 50% resolution. (The red pixels on the color array are ignored.) The second dichroic surface is used to isolate red light at the second exit plane of the prism. A monochrome CCD sensor acquires the red plane at full resolution. Finally, a third monochrome CCD is used to image the near-ir light exiting the rear of the prism. 5
Figure 6. RGB/CIR Spectral Configuration By implementing a similar Bayer pattern de-mosaic algorithm, the data from this configuration can be used to create a full resolution RGB image (in addition to a CIR or IR image). The resolution of this RGB image is somewhat lower that that acquired with a 3-CCD RGB camera. Keep in mind however, that it is actually higher resolution than the image acquired with a single-chip color camera because in the RGB/CIR configuration the red image is acquired at full resolution (rather than the 25% resolution from a color sensor). The color band that has the least raw resolution in this configuration is the blue band. Fortunately, for many applications, the amount of information in the blue band is fairly limited so the effect of the loss in resolution is minimized. The passbands of our standard RGB/CIR configuration are listed in Table 1 below. Passbands for the Red and IR channels can be customized. Table 1. Passbands for MS 3100/4100 Standard RGB/CIR Configuration Band Center FWHM Wavelength Blue* 460 nm 45nm Green* 540 nm 40 nm Red 670 nm 40 nm IR 800 nm 65 nm * Values based on sensor manufacturer's spectral data 6
POLARIZATION CONFIGURATION For polarization imaging, the prism surfaces are fabricated with neutral beam splitter coatings. The first coating surface reflects 30% of the light and transmits 70%. The second coating provides a 50% transmittance and 50% reflectance. This combination results in splitting the incoming broadband light into three components with equal spectral and spatial content. The trim filters are linear polarization filters with >99% polarization efficiency. The filters can be oriented per customer specification. Commonly the filters are oriented at 0, 45, and 90 degrees or three equal angular spacings. The polarization camera can be configured with either color or monochrome sensors for each channel. The CCD sensors for our polarization cameras can be RGB color or monochrome. If color sensors are specified, the data output will be in the form of the raw Bayer pattern for each sensor. Each color planes can be expanded to a full color image on the receiving PC platform by using a Bayer demultiplexing algorithm in the frame grabber or software. The camera s internal Bayer demultiplexing engine can demultiplex any individual channel to provide a real-time preview during image acquisition. Future products will support Bayer demultiplexing of three sensors simultaneously. Figure 7. Polarization Configuration 7
CUSTOM MULTISPECTRAL The discussion above has shown that the spectral content of the light arriving at each of the three imaging sensors in MS cameras is determined by two factors: 1) the dichroic coatings on the prism faces, and 2) the optical trim filters between the prism exit plane and the imaging array. In custom configurations, it may be necessary to adjust one or both of these factors in order to achieve the desired spectral configuration. The easiest and least expensive adjustment is achieved by customizing the pass bands of the optical trim filters. As long as the targeted imaging bands fall within the larger spectral bands created by the prism, the target band can be selected by using the appropriate trim filter. It should be noted that customization of the trim filters is a factory operation. Due to the precision mechanical assembly and alignment process, field exchange of filters is not supported. If the targeted imaging bands do not fall within the pass band of each of the three imaging channels of the prism, it may be necessary to fabricate prisms with a customized dichroic coating in order to adjust the pass bands of the prism assembly. This process incurs more expense and lead-time, as a specialized coating run is required. Prices will be quoted based on the specifications of each specific project. GSI maintains two stock prisms that are used in our standard configurations. The stock prism components may be assembled in three possible configurations. The transmission bands of each of these configurations are specified in Table 2 below. There are a limited number of additional prism components in stock. Call and check for availability. Prism Configuration Table 2. Stock Prism Components Dichroic Coating #1 Cut-on/Cut-Off Wavelength* Dichroic Coating #1 Cut-on/Cut-Off Wavelength* CIR 605 +/- 8 nm 730 +/-10 nm RGB 605 +/- 8 nm 575 +/-8 nm * - All transitions specified for slope < 5% In building a custom multispectral configuration, several combinations of coated substrate glass and prefabricated bandpass filters can be used for trim filters. When specifying the characteristics of an optical filter, the center wavelength where the filter s peak transmission occurs is quoted, along with the width of the pass band at the point where the transmission has been reduced by 50%. This is known as the full width, half maximum (FWHM). We have a variety of trim filters with center wavelengths ranging from 450-900 nm and passbands (FWHM) of 10, 40, and 70 nm. Since it is very difficult to control the exact transmission characteristics of an optical filter during the fabrication process, accuracy of the specified passband is usually quoted as +/- n nm. When specifying a custom spectral configuration, it is always advantageous to choose passbands that can be accomplished with stock filters. Custom optical filters can be fabricated, but it can be a very expensive process. To summarize the considerations in specifying a custom multispectral configuration they are as follows: 1) Targeted imaging bands must be isolated within the pass band of each prism channel. The only exception to this rule is when a color CCD array is going to be used in one of the channels to pick up two or more of the visible color bands. In this case, the color bands must fall within a single channel of the prism configuration. 8
2) If possible, stick with configurations that can take advantage of GSI s stock prism configurations. Custom coating runs add additional expense and lead time delays. 3) When selecting the center wavelengths and passbands for your project, keep in mind that the narrower the band, the less light there is available to the imaging sensor. In addition, the reduced sensitivity of the sensors at the limits of their range further diminishes the available signal. The reduced light levels may not be a problem for applications that will image in sunlight or where the illumination source can be controlled. 4) As much as is reasonably possible, the relative quantity of light available to each imaging sensor should be balanced. The final balancing of the imaging channels is accomplished with gain adjustments in the camera electronics. However, performance is optimized if the light levels arriving at the sensors are similar. For example, if a 70 nm passband is used on one channel with ample ambient light and another channel is operating with a 10nm bandpass near the limits of the sensor s sensitivity, it will be difficult to adjust the two channels so that each is using the full dynamic range of the sensor. The relative sensitivity of the sensor at various wavelengths is a significant factor in achieving signal balance. (Note: Gain, offset, and exposure time can be independently adjusted for each of the imaging channels.) 5) Remember that filter replacement is a factory operation. Field exchange of filters is not supported. Additional Configurations There are a number of additional possibilities for custom camera configurations. A few examples are listed below to spark your imagination. Multi-Wavelength Fluorescence Imaging In fluorescence imaging, typically one wavelength the excitation wavelength is used to illuminate the target and cause the fluorescence. The fluorescence caused by this excitation generally occurs at one or more wavelengths that are different from the excitation source. In capturing the fluorescence images, it is desirable to filter for only the narrow fluorescence band, using deep blocking to exclude excitation light from the fluorescence image. Using a MS multichannel camera, one channel might be filtered to capture an image based on the excitation illumination while the other two channels could be filtered for the fluorescence wavelengths. Broadband Separations In some applications, there is an interest in acquiring simultaneous images from separations of light based on criteria other than wavelength. By using a broadband beam splitter instead of dichroic separation, the same range of wavelengths can be delivered to two different channels. The light delivered to each channel can be split on a percentage basis rather than wavelength separation. This can be of particular interest when the targeted imaging bands lie very close together. If the bands are too close together for efficient separation with dichroics, the available light can be split between the two channels and then filtered for the appropriate band in each channel. Optimum Quality Customized for Your Needs Geospatial Systems, Inc has implemented a number of proprietary, features in our optics to insure top performance. Internal light baffles block stray light and a proprietary coating on nontransmissive surfaces absorbs stray light. The mounting assembly for the optical system is fabricated from metals with thermal expansion coefficients matched to the prism glass. This insures, stable, stress-free operation, and rock solid alignment even when subjected to temperature variations. GSI s superior imaging systems offer you the opportunity to acquire high quality digital imagery based on controlled spectral content. We offer a wide variety of stock and custom configurations that can address the needs of wide ranging applications. Our technical staff will be happy to 9
discuss your requirements with you in order to develop the most efficient and cost effective means to accomplish your goals. Contact us today for more information or to place an order. How to Reach Us: Geospatial Systems, Inc 125 Tech Park Dr Rochester, NY 14623 Phone:585-427-8310 Fax: 585-427-8422 info@geospatialsystems.com www.geospatialsystems.com 10