Now that we have covered the human eye, it seems natural to describe an engineered eye, otherwise known as a camera. People have built and used cameras for hundreds of years, starting with a camera obscura that allows light to pass through a pinhole and onto a surface that contains the real image. Figure 4.31 shows an example that you might have constructed to view a solar eclipse. (Recall the perspective transformation math from Section 3.4.) Eighteenth-century artists incorporated a mirror and tracing paper to un-invert the image and allow it to be perfectly copied. Across the 19th century, various chemically based technologies were developed to etch the image automatically from the photons hitting the imaging surface. Across the 20th century, film was in widespread use, until digital cameras avoided the etching process altogether by electronically capturing the image using a sensor. Two popular technologies have been a Charge-Coupled Device (CCD) array and a CMOS active-pixel image sensor, which is shown in Figure 4.32(a). Such digital technologies record the amount of light hitting each pixel location along the image, which directly produces a captured image. The costs of these devices has plummeted in recent years, allowing hobbyists to buy a camera module such as the one shown in Figure 4.32(b) for under $30 US.