Measurements by Laser can be done in various ways to measure distances or displacements without physical contact. In fact they allow for the most sensitive and precise length measurements, for extremely fast recordings (sometimes with a bandwidth of many megahertz), and for the largest measurement ranges, even though these qualities are usually not combined by a single technique. Depending on the specific demands, very different technical approaches can be appropriate.
They find a wide range of applications, for example in architecture, inspection of fabrication halls, criminal scene investigation (CSI), and in the military.
TECHNIQUES FOR MEASUREMENTS BY LASERS :
Some of the most important techniques used for measurements by Laser are –
- Triangulation is a geometric method, useful for distances in the range of ≈ 1 mm to many kilometers.
- Time of Flight Measurements (or pulse measurements) are based on measuring the time of flight of a laser pulse from the measurement device to some target and back again. Such methods are typically used for large distances such as hundreds of meters or many kilometers. Using advanced techniques, it is possible to measure the distance between Earth and the Moon with an accuracy of a few centimeters. Typical accuracies of simple devices for short distances are a few millimeters or centimeters.
- The phase shift method uses an intensity-modulated laser beam. Compared with interferometric techniques, its accuracy is lower, but it allows unambiguous measurements over larger distances and is more suitable for targets with diffuse reflection.
Note that the phase shift technique is sometimes also called a time-of-flight technique, as the phase shift is proportional to the time of flight, but the term is more suitable for methods as described above where the time of flight of a light pulse is measured.
- For small distances, one sometimes uses ultrasonic time-of-flight methods, and the device may contain a laser pointer just for getting the right direction, but not for the distance measurement itself.
- Frequency modulation methods involve frequency-modulated laser beams, for example with a repetitive linear frequency ramp. The distance to be measured can be translated into a frequency offset, which may be measured via a beat note of the sent-out and received beam.
- Interferometers allow for distance measurements with an accuracy which is far better than the wavelength of the light used.
Distance measuring by laser :
Laser Distance Measuring Tools, also known as “Electronic Tape Measures”, “Laser Distance Finders” or “Digital Measuring Devices”, offer true laser light (not sound) for the most accurate measuring.
You simply point the laser distance measuring devices dot at a target like a wall, a house, a utility pole, nearly any object, and the device will show the distance on its LCD screen. These measuring devices will show multiple units of measure, you simply cycle through the units till you get to the units of measure you want to measure in, and then start taking measurements
. You can even cycle through the units of measure after taking a measurement.
These devices can also be put up against a wall and they measure from the back of the unit so that you get true wall-to-wall distance measurement. Same goes with ceiling measurements, lay the device on the backside, point it up and it will measure from the floor to the ceiling. These measuring devices are true time savers and are very accurate.
Compared with ultrasonic or radio and microwave frequency devices (radar), the main advantage of laser distance measurement techniques is that laser light has a much smaller wavelength, allowing one to send out a much more concentrated probe beam and thus to achieve a higher transverse spatial resolution. Another advantage that an optical bandpass filter makes it possible to very effectively remove noise influences at other optical frequencies.
MEASUREMENT BY NUMERICAL APERTURE :
the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. A laser beam focused by an objective lens is introduced to the flat surface of a SIL, emitted through the spherical surface, and then detected. In this way the divergence of the laser beam is reduced, and as a result the detection efficiency of the laser power increases.
For many laser applications, information on irradiated laser power is important. However, direct measurement of laser power through a high numerical aperture objective lens is difficult in a laser microscope.
FIG :MEASUREMENT BY LASER:
QUESTION AND ANSWER BASED ON MEASUREMENT BY LASER :
Q 1) WHAT ARE VARIOUS ISSUES ?
As essentially all other measurement techniques using lasers, laser distance measurements can be affected by laser noise. Other noise-related issues can arise from detection noise, stray light, and speckle effects.
The targets can have very different reflection and scattering properties. Problems can arise for very low reflection or for specular reflections.
Note that range finding with lasers can raise serious laser safety issues, particularly when intense pulses from Q switched laser are used. The related hazards can be strongly reduced by applying eye safe laser.
Q 2) EXPLAIN WORKING ( EFFECTIVE) f- NUMBER ?
The f-number describes the light-gathering ability of the lens in the case where the marginal rays on the object side are parallel to the axis of the lens. This case is commonly encountered in photography, where objects being photographed are often far from the camera. When the object is not distant from the lens, however, the image is no longer formed in the lens’s focal plane , and the f-number no longer accurately describes the light-gathering ability of the lens or the image-side numerical aperture. In this case, the numerical aperture is related to what is sometimes called the “effective f-number.” A practical example of this is, that when focusing closer, with e.g. a macro lens, the lens’ effective aperture becomes smaller, from e.g. f/22 to f/45, thus affecting the exposure.
The working f-number is defined by modifying the relation above, taking into account the magnification from object to image:
- Q 3) WHAT IS LASER RADAR ?
- Ans 3 ) A laser radar is a device which uses one of the distance measurement techniques as described above, and scans the direction of the distance measurement in two dimensions. This allows the acquisition of an image, or more precisely a depth profile of some object, as required e.g. in robotics. For acquiring such depth profiles at a higher rate, there are sensor chips similar to CCDs (charge-coupled devices) with internal electronics to detect phase shifts, so that the distance for each pixel can be measured simultaneously. This allows for rapid three-dimensional imaging with very compact devices.