Today, most Infrared CCTV cameras come with LEDs of 850nm wavelength. The infrared wavelength of 850nm is an optimum level. It is the wavelength nearest to the visible light spectrum at which we can also capture images in complete darkness. The imaging sensors of cameras are most sensitive at this infrared wavelength. As wavelengths go higher than 850nm, distortions in picture quality start to increase. Therefore, cameras designed to operate at wavelengths higher than 850nm include additional technology to overcome these distortions. As a result, cost of these cameras is also high, and they are thus less common. However, 850nm infrared LEDs produce a faint red glow. For some applications, like covert surveillance, this is not acceptable. For secret surveillance, many companies make 940nm IR LED cameras. The 940nm IR LEDs do not produce any light visible to the human eye. However, the reach of 940nm infrared is around 40% shorter than that of similar 850nm IR cameras.