Using Laser Distance FindersBy Brian Rhodes, Published Jun 04, 2012, 12:00am EDT
Calculating correct distances can be a pain but are an essential part of properly estimated projects. For years, the tape measure has been a constant companion on site surveys, but even the simple task of taking measurements can be a time consuming, two man job - especially for longer distances. Does a better alternative exist? In this note we review laser distance meters, a simple handtool that we have found to be a great alternative to tape measures when taking measurements in the field.
The time it takes light to travel can be used to calculate distance. The longer it takes for a laser to bounce off of a remote target and return to the sending point, the farther away the target is located.
Laser distance finders, or 'rangefinders' are handheld, battery powered devices that use this principle to measure distance. These units emit a low power laser matched to a sensitive collector that measures minute amounts of reflected light. A very precise timer measures this loop many thousands of times in a second, and the time value is converted to read as a distance measure on the meter's display.
While a wide variety of models exist, features common to most units can also be found in the example below:
Features like extended measurement distance (lengths exceeding 650' are common), area measurements, memory capacity, and enclosure durability all influence final cost. However, for most security installers the basic, most inexpensive models are sufficient.
Quick: The speed that measurements are taken might be the biggest advantage of laser distance finders. When compared with the process of unreeling measuring tape, stepping off a distance with a wheel, or counting ceiling tiles, the laser unit is almost instantaneous. The term 'point and shoot' is often used to describe the process required to collect a measurement, and several measurements covering hundreds of feet can be taken in a minute or less.
Lightweight and small: The physical dimensions of a distance finder are small, approximately the same size as a multimeter. Devices can be easily carried on belt clips or in tool bags, and can be operated using only one hand.
Inexpensive: Laser distance finders can be purchased for under $100, and while more expensive than traditional tape measures, models are not so expensive they cannot be purchased as a common hand tool.
Accurate enough for most measurements: Early versions of laser distance finders were prone to being wildly inaccurate, but current models are accurate and precise to fractions of an inch. For example, this 650' distance model is accurate to +/- 0.04 inches at maximum measured length. The more significant concern for modern models is angularity error the operator introduces in holding the unit 'off-target'. In any case, for the rather coarse measurements taken during a site survey, the laser distance finder is sufficient for most video security work.
Hard to see in bright sunlight: The most common complaint of using laser distance finders are that finding the 'laser dot' can be difficult in brightly lit areas. While the ambient brightness and reflectivity do not affect the accuracy of the tool, it is easy to lose the dot location in sunlight flooded areas. A variety of tools exist to help reduce this problem, including 'laser enhancement glasses' that make the dot appear more intense in sunlight.
Hard to see at extreme distances: Along with brightness, successfully locating a 'laser dot' at extreme lengths can be difficult. The diameter of the laser dot is often 1/4" or less, and over hundreds of feet it can be very difficult to locate. Some 'high end' distance finders include a built-in camera with zoom [link no longer available] to help with this. Some users may opt to fabricate a 'target', like we demonstrate in the video below, that simply provides a dark, high contrast target to bounce a beam from in the field.
Humidity, heat can affect accuracy: While modern units have been engineered to overcome the weaknesses of early models, certain environmental situations can affect unit performance. For example, heat influences beam divergence (expanding beam diameter) over distance and can distort the timing of beam travel. While this distortion may be an extremely minute amount,even a small visual disturbance can result in a significant distance error. Likewise, high humidity (the presence of moisture in the air) not seen with the naked eye can sometimes contains enough water vapor to introduce meter error. The best approach to take to prevent these errors are to read the unit manual and apply the meter only within environment parameters defined by the manufacturer.
Not all units are create equal: As with any manufactured device, the quality of components affect usability. Modern units are built with 'pulsed emitters' designed to minimize measurement error, but older 'fixed emitter' technology is still used in lower-cost alternatives. Because 'laser distance finder' is a generic term, care should be taken to select a unit with a proven history of use. This risk is often most significant when specifying units with extreme maximum distances (>800 feet) and multi-function units (Ultrasonic stud finder/laser distance finder combos) Fortunately, units built with good technology can be found even at the low-end of pricing. Simply reading product reviews on these units can go a long way in choosing the right unit.
Here is a quick video that introduces this tool and shows basic operation. While exact operation may vary from unit to unit, the function below is common to most tools in the category:
The measuring wheel is the most common alternative to laser meters. In a future post, we will review measuring wheels and compare.
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