Focal Plane and CCD Array
The focal plane of an aerial camera is the plane where all incident rays coming from the object are focused. The focal plane is where the film of a film-based camera is placed. With the introduction of digital cameras, the focal plane is occupied by the CCD array, replacing the film.
A digital camera like the ones we use at home is called a “digital frame” camera just to distinguish it from other designs of digital cameras such as “push broom” cameras. Digital frame cameras have the same geometric characteristics as the film camera that employs the film as the recording medium.
A digital frame camera consists of a sensor that is a two-dimensional array of charge-coupled device (CCD) elements (CCD is also called pixel). The sensor is mounted at the focal plane of the camera. When an image is taken, all CCDs of the sensor are exposed simultaneously, thus producing a digital frame. Figure 4.3 (from Wolf, page 75) illustrate how a digital camera captures an area on the ground that falls within the lens' field of view (FOV).
The size of a digital camera is measured by the size of its sensor. The higher number of CCDs (pixels) in the sensors, the bigger and more expensive the camera is. If a camera has a sensor with 4000 pixels by 4000 pixels, it is called a 16 megapixels camera. That is because it has 16,000,000 pixels. UAS imaging productivity, i.e. how many acres the UAS can cover in an hour, depends on the sensor size, battery life, and the lens focal length. The article "DJI Phantom 4 RTK vs. WingtraOne" clearly illustrates the difference between UAS productivity based on sensor and UAS capabilities. In that article, you will also learn about some fundamental capabilities that we usually expect from a mapping drone.
The lens for a mapping camera usually contains compound lenses put together to form the lens cone. The lens cone also contains the shutter and diaphragm.
The lens is the most important and most expensive part of a mapping aerial camera. Cameras on board of the UAS are not of that level of quality, as they were not manufactured to be used as mapping cameras. Mapping cameras are called metric cameras, and are built so that the internal geometry of the camera holds its characteristics despite harsh working conditions and changing operational environments. Lenses for cameras on board of the UAS are small in size and lighter in weight. They are also less expensive than standard mapping cameras. Lenses for mapping cameras should be calibrated to determine the accurate value for focal length and lens distortion (imperfectness) characteristics.
Shutters are used to limit the passage of light to the focal plane. The shutter speed of aerial cameras typically ranges between 1/100 and 1/1000 seconds. Shutters are of two types: focal-plane shutters or the between-the-lens shutters. The latter one is the most common shutter used for aerial cameras. Most digital camera shutters are designed according to two mechanisms: the leaf shutter (also called mechanical or global shutter or the dilating aperture shutter) or the electronic rolling shutter (curtain or sliding shutter). The leaf shutter exposes the entire sensor array at once, while the rolling shutter exposes one line of pixels at a time. For aerial imaging from a moving platform such as a UAS, leaf shutter is recommended because it minimizes image blur. To understand the shortcoming of the rolling shutter, watch this video.
It is important to know which shutter is used for your camera as most processing software including Pix4D provide correction for the rolling shutter effect. However, the software does not correct for it automatically, and you will need to activate that option before you start processing the imagery.
More information on different types of shutter mechanisms can be found on Wikipedia's Shutter (photography) page.
- Chapter 3 of Elements of Photogrammetry with Applications in GIS, 4th edition