Enhancing the home automation experience

When it comes selecting the appropriate CMOS for a home automation IoT device what do you need to take into account? Radhika Arora explains

ensors are at the heart of the are called zoom lenses, those with the lens, and the of the Internet of Things (IoT) Ixed focal length are prime lenses. lens. Larger (smaller f- revolution and most Within Ixed focal length, lenses of number) results in a shallower DOF. Sapplications will deploy multiple shorter focal length are called wide- Closer focus distance results in a sensors including an image sensor. angle lenses (14 to 35mm, 114 to shallower DOF, which is appropriate for The more compelling home automation 64°). Longer-focal-length lenses are artistic imaging. It focuses on the products tend to deploy referred to as long-focus lenses subject at hand and dims out the which are commonly based around a (85mm to >300mm, 30° to <1° FOV) background, highlighting the main CMOS image sensor and this, coupled and are used to magnify distant object. For a given f-number, increasing with sophisticated computer vision objects. the magniIcation, either by moving the algorithms, look set to become the For example, home security closer to the subject or using ‘brains’ of the smart home. cameras which make up a large a lens of greater focal length, Field of View (FOV) is what is visible portion of the IoT typically have wide decreases the DOF; decreasing through the camera at a particular angle Ixed focal length lenses with magniIcation increases DOF. position and orientation and is which to monitor and report on speciIc Aperture is an indicator of how controlled by the focal length. This is regions of interest. much light can enter the lens, similar the distance from the point of to the iris of the eye. Consider two convergence to the focal plane and lenses with the same size optical More compelling varies with the wavelength of light. For Another metric to consider is depth of format. A wider aperture (f/1, for example, the focal length of blue light Ield (DOF). It represents the distance home automation example) will allow more light to enter (450nm) is slightly shorter than it is between the nearest and farthest products tend to than a narrower aperture (f/12, say). A for red light (620nm). objects and is determined by three deploy cameras. wider aperture results in a faster Lenses with variable focal length factors – aperture size, distance from Photo: Alamy resulting in capturing

22 27 October 2015 www.newelectronics.co.uk RESEARCH & DEVELOPMENT SENSORS

high speed motion with less blur. More values are available, they are present in the image – noise shows up light also means less graininess in low combined to create a linearised value as graininess in the image. There are light. If low light performance is critical for each ’s response. Alternatively, two primary ways in which a sensor’s to the application then a lower f the sensors have the option to output SNR can be increased: decreasing number lens is important. two separate streams of data noise; or increasing the signal, thereby If we consider a home security representing the two varying optimising the response of the sensor application, the f/1.8 lens is the most exposures. These can then be in terms of quantum efIciency (QE). common option. Superior image quality processed off chip. QE represents the percentage of especially in darker environments is Initially, DIY devices have used photons that get converted into key for most home automation sensors with a standard dynamic electrons. For low-light use cases, devices. In fact, low light performance range (54dB to 70dB) but as devices where subtle differences in light levels is one of the strongest selling points have got ‘smarter’ and use cases must be captured, less than 1mV of for an image sensor. more diverse, there is a requirement noise on a chip can be perceived as for higher dynamic range to be noise in the image. A noisy Dynamic range supported. In the indoor environment, environment can easily dominate a For an image sensor, dynamic range there is a lower probability of large low-level signal generated in low-light (DR) provides the range between the differences in lighting in a single or shaded conditions. Peak SNR, as scene. However, use the same device seen on some high performing CMOS in an outdoor environment and there is image sensors in the IoT space, runs a much higher chance of having a between 39dB and 41dB. brightly lit area coupled with shadowed Some home automation products areas in the same scene. are beginning to adopt RGB-NIR image Today, cameras are expected to sensors in their applications. RGB-NIR compensate for sudden changes in is a pattern of colour Ilters on the lighting, such as a door opening, or the image sensor’s pixel array that turning of a light switch. An HDR image individually collect red, green, blue sensor provides the capability of and near infrared (NIR) photons. The seeing the door opening, or the light image quality as seen from these being switched on and off, while sensors provides good colour maintaining clarity in image quality. reproduction during the day when the Such technology makes it much easier IR is low and at night with NIR LEDs to track people and objects, identify actively used and only faces in the hardest of lighting images are taken. These sensors conditions. eliminate the need for a mechanical IR cut Ilter. This reduces overall cost Low light performance and cuts down on Ield failures High quality imaging in low light is a resulting from the mechanical key selling feature for IoT applications. operation. When choosing the sensor for camera When streaming over data limited systems that must produce high- wireless protocols, the video typically quality images under low-light goes through compression. The brightest and darkest detailed parts of Cameras are at the conditions, there are several higher the compression, the more an image that can be captured heart of the IoT parameters to consider such as complex the processing, leading to simultaneously. Image sensors revolution. modulation transfer function (MTF) and higher power consumption so while typically have a dynamic range Photo: Alamy Signal to Noise Ratio (SNR). 18Mpixel cameras may sound between 54dB and 70dB. Higher MTF is a common way to quantise compelling, power tradeoffs force dynamic range is derived through the ability of a sensor to provide cameras for most IoT applications to image processing, which can happen sharpness of an image. MTF in the be in the 2Mpixel to 5Mpixel range. in the sensor or in the image visible light spectrum is quite Cameras are at the heart of the IoT processor. Some sensors in the consistent, problems tend arise at revolution, giving rise to new market support high dynamic range longer wavelengths. Lower MTF limits categories of devices and applications (HDR), going up to 105dB. the resolution of the system making and advances in CMOS image sensor In HDR mode, sensors capture two small detail not very obvious. technology are helping to drive that exposures sequentially within the SNR is another key factor revolution. same frame by maintaining two influencing the ability of a sensor to separate read and reset pointers deliver a useful image. The higher the Author profile: interleaved within the rolling shutter SNR, the better the image quality. SNR Radhika Arora is product line manager readout. As soon as a pixel’s two gives the ratio of signal to noise (IoT) with ON Semiconductor.

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