What Is a Lighting Reflector?
A Lighting Reflector is a device that detects the presence of an object by projecting light from a light emitter and a light receiver facing in the same direction and detecting the light reflected by the object with a light receiver.
A mechanism that uses light to detect an object is generally called a photointerrupter, and refers to a device that has the ability to detect the presence or absence and position of an object by light using a set of light emitters and photodetectors.
Photointerrupters are classified into transmission and reflection types. Transmission type photointerrupters detect objects by intercepting light when it passes between the light emitter and receiver.
Reflective photointerrupters detect light emitted by a light emitter that is reflected by an object and detected by a photodetector.
Since the two are usually distinguished by the term photointerrupter for the transmission type and lighting reflector for the reflection type, this article will focus exclusively on the latter type.
Uses of Lighting Reflectors
Lighting Reflectors are mainly used to detect objects at close range. They can also be used to detect black-and-white patterns by utilizing the fact that the reflectance of light differs depending on the surface color of the object, or to measure distance, since the signal strength changes depending on the distance between the object and the lighting reflector.
A specific example is the ability to determine whether or not an object is in a given position. This is the case, for example, with the detection of the state of media installation in a disc player. In encoders used in various devices, a circular slit plate with a slit engraved on it is attached to the axis of rotation, and a photo reflector is also used in a mechanism that detects the amount of rotation by counting this slit.
Since there is no object in the chipped portion of the slit, the number of chipped portions can be counted from the light-receiving part’s signal. An example of detecting color differences is when a self-propelled robot is used as a sensor in a mechanism that detects the path when it moves along a white guide line.
Another use of a photo reflector is to measure rough distances, an example being the detection of ink or toner remaining in a printer.
Principle of Lighting Reflectors
Lighting Reflectors consist of a near-infrared LED light-emitting element and a phototransistor or photodiode light-receiving element, both of which are oriented in the same direction. Light emitted from the light emitting element is reflected by the object to be detected, and the light receiving element detects the reflected light to detect the object.
The intensity of the reflected light depends, in part, on the distance between the object and the lighting reflector. The closer the object and lighting reflector are, the stronger the reflected light will be, so the distance between the object and photointerrupter can be roughly determined from the strength of the reflected light.
In addition, since the reflectance of light varies depending on the color and density of the object, detecting the intensity of the light can also be used to detect differences in color and shading.
Other Information on Lighting Reflectors
Points to Keep In Mind When Employing a Reflective Photo-Micro Sensor
1. Ambient Light
Since the light-receiving element of a lighting reflector is structurally designed to face outward, it is necessary to take into consideration the fact that ambient light tends to penetrate into the light-receiving part.
Usually, a visible light cut filter that blocks light with a wavelength of 700 nm or less is installed in the light-receiving part of lighting reflector to reduce the effect of ambient light, but even this does not completely prevent it.
2. Background Influence
Background influence means that the light emitted by the light emitter is reflected by a background object, even if there is no object to be detected, causing false detection by the light receiving element. For example, if a piece of paper passes over a stainless steel or galvanized frame, and the paper’s passage is detected, it can happen that the light-receiving signal level without the paper is larger than the light-receiving signal level when the paper is detected.
Therefore, it is desirable for the background to be pitch black or have a large space available.
3. Light Reflectance of the Detected Object
It is important to note that the output level of reflected light varies greatly depending on the type, distance, and size of the object to be detected. Originally, the photocurrent of a reflective photo-micro sensor only provides a current level of several tens to several hundred µA even when detecting an object, so it is necessary to recognize that the output level of the light receiving element itself is low.
It must be noted that even when the surroundings are dark, dark currents and leakage currents flow through the photosensor, and these can reach 10 µA or more with a temperature rise, and these noise levels cannot be ignored relative to the signal level. Particular attention should be paid to objects with low reflectance, as the ratio of signal to noise will be very small.
4. Detection of Small or Transparent Objects
If the object to be detected is small or transparent, the reflected light is weak, so the output signal level of the photodetector is small and may be lost in the noise. It is important to check the specifications of the photointerrupter with regard to the size of the object to be detected.