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The Extech LT40 LED light meter is useful for measuring and optimizing environmental light levels in a wide range of environments, including office buildings, manufacturing plants and warehouses, university campuses, schools, hospitals, and many other locations. As OSHA regulates workplace lighting, light meters such at the LT40 can verify regulatory compliance, as well as state and local public safety codes and ordinances. The LT40 can also measure standard light sources such as fluorescent, metal halide, high pressure sodium, and incandescent lighting, up to 400,000 lux (40,000 Fc). The LT40 comes with a built-in sensor with protective cover, 2 AAA batteries, storage pouch, and user manual.

The way to distinguish among the two types of lenses is to look at the relative thickness of two parts — the center and the edges. Converging lenses are thicker in the middle than they are at the edges, while diverging lenses are thicker at the edges than they are in the middle. Converging lenses include those that are biconvex (curved outward on both sides), plano-convex (flat on one side and curved outward on the other side), and convex meniscus (curved inward on one side and outward on the other side more strongly). Diverging lenses include those that are biconcave (curved inward on both sides), plano-concave (flat on one side and curved inward on the other side), and concave meniscus (curved inward on one side and curved outward on the other side less strongly).

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401025 is a Foot Candle/Lux Meter which measures light level to 5000FC (foot-candles) and 50,000 Lux with a 5% accuracy using a precision photo diode and color correction filter. Meter features fast/slow response time and an analog output (1mV per count) for capturing readings to a recorder. Comes complete with 9-Volt battery.

The SI units of each of these quantities is the inverse meter [m−1], which is given the special name diopter [D]. The power of eyeglasses and contact lenses are most commonly expressed in this unit. The greater the power a pair of eyeglasses or contact lenses has, the worse is the vision of the person prescribed them. Converging lenses, like those normally found in reading glasses, have positive refracting powers. Diverging lenses, like those worn by people with myopia or near-sightedness, have negative refracting powers. A flat piece of glass, which is essentially the same as no lens at all, has a power of zero. The eye has a power of 50 D when focusing on distant objects.

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Extech’s pocket-sized autoranging Light Meter offers wide light intensity ranges measuring up to 40,000Fc (400,000Lux) with high resolution to 0.01Fc (0.1Lux). Ideal for indoor lighting tests and for checking security and safety illumination in parking garages, night time ATM areas, stairwells, landings, and hallways.

This compact and ruggedly designed meter has a wide measurement range to 99,990 Foot Candles in 5 ranges and 999,990 Lux in 5 ranges with 0.001Fc/0.01Lux resolution. Features include: Store and Recall up to 50 measurements with relative or real time clock stamp, Luminous intensity (candela) calculation, a ripple function that excludes stray light from the primary light source measurement, Multi-point average function, Timed Hold, Relative measurement in absolute value or % deviation, Comparator function with high/low alarms, and Auto Power Off with disable. Complete with built-in stand, light sensor and protective cover with 36” (0.9m) coiled cable, protective holster, 6 AAA batteries, and carrying case.

The Extech UV510 Light Meter measures UVA light radiation from natural and artificial light sources. It offers a backlit dual display, cosine corrected sensor, two selectable ranges, zero function, min/max recording, display hold, and auto power off. The compact size with handy wrist strap gives you convenience in taking quick UV reading wherever you go.

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The LT45 LED light meter is useful for measuring and optimizing environmental light levels in a wide range of environments, including office buildings, manufacturing plants and warehouses, university campuses, schools, hospitals, and many other locations. OSHA regulates workplace lighting because illumination must be adequate for assigned tasks, and light meters are used to verify regulatory compliance, as well as state and local public safety codes and ordinances. Use of LED lighting is on the rise because it is more efficient to operate, it emits stable, high quality illumination, and the technology is dropping in price. The LT45 comes with a 9-volt battery, protective sensor cover, hard case, and user manual.

Nearly every lens has a line of symmetry down its center (which may or may not be its geometric center, but usually is). Given a standard, simple lens like eyeglasses, contact lenses, or the lens in a movie projector, there is an obvious axis of symmetry about which the lens could be rotated and not have any effect on the image produced. Such an axis is called the principal axis. To keep life simple, when parallel rays are shown entering a lens, they are often all drawn parallel to the principal axis. Such rays are said to be paraxial. As I have just said, this is merely done for convenience. Actually, for an ideal lens, parallel rays always converge on or diverge from a point. (The aberrations of a real lens from an ideal lens are dealt with in another section of this book.) For a lens with spherical symmetry, the collection of focal points for any group of parallel rays form a focal plane. This behavior is best illustrated with a series of pictures.

Rays of light parallel to each other converge on the focal plane after passing through a converging lens as illustrated in this diagram.

The origins of the word lens can be traced back to the 17th century. Scientists at the time wanted a word to describe the shape of the glass pieces used in microscopes, telescopes, magnifying glasses, and reading glasses. For some unknown reason they decided against the strictly descriptive term "biconvex" (curved outward on two sides) and instead chose to name them for the small, flat beans they resemble — lentils. Lens lentis is the Latin word for lentil. It's good word for English speakers since few of us associate lenses with lentils and not all lenses are lentil shaped.

The Extech SDL470 is a UVA/UVC Light Meter and Data logger. The SDL470 can measure both long wave and short wave UV irradiance under UVA and UVC light sources respectively. This meter features a cosine correction filter, data logging to Excel® format, offset adjustment for relative measurements, Type-K/J thermocouple input for temperature measurements (probe sold separately), record/recall modes, min and max readings, and data hold plus auto power off with disable function. This product comes with a built-in tilt stand, SD memory card, UVA light sensor with protective cover, UVC light sensor, hard case, universal AC adaptor, and 6 x AA batteries.

The LT250W is a compact light meter with Bluetooth® connectivity, which allows building and maintenance professionals to view and share light intensity data directly from a mobile device using the free ExView app. With the capability to capture data, display trends, set visual and audio alarms, and create and send reports, users can gain deeper insights on workplace comfort and building health. The LT250W is part of the Extech 250W Connected Series of products that can improve your ability to evaluate problems and report on them from the field. This allows you to improve the speed and quality of decisions to address critical issues immediately.

Every ray from an object (O) will continue undeviated if it passes through the center of a lens. This one path will be the same for each lens individually as well as for the combination of lenses. (The lenses are assumed to be so close together that their centers essentially coincide.)

The ray through the focus of the first lens (F1) will emerge parallel to the principal axis. The image formed will lie at the intersection of the two rays drawn. Since it is not the final image, it is indicated with the symbol I*.

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What makes a lens different from any other transparent object is its ability to focus light. A focus is a meeting point. The origins of this word are also Latin. Focus is the Latin word for fireplace. Like the word lens, the word focus first started to appear in scientific writing in the 17th century. Many people are familiar with the ability of magnifying glasses to concentrate sunlight to the point where it can burn paper. It is uncertain whether the scientist of this time were referring to the fire produced by this method or the fact that the fireplace was the meeting point (or focus) of the home at the time.

The SDL400 records data on an SD card in Excel format. Wide range to 10,000Fc or 100kLux. Cosine and color-corrected measurements. Utilizes precision silicon photo diode and spectral response filter. Offset adjustment used for zero function to make relative measurements. Adjustable data sampling rate. Stores 99 readings manually and 20M readings via 2G SD card. Type K/J Thermocouple input for high temperature measurements. Large backlit LCD display. Record/Recall Min, Max, Data Hold and Auto power off. Complete with 6 x AA batteries, SD card, hard carrying case and light sensor with protective cover.

If you are a vertebrate with eyes, then you have lenses. The same goes for octopus, squid, and cuttlefish (which are all cephalopods). Common optical devices that use lenses include eyeglasses, contact lenses, magnifying glasses, cameras, projectors, telescopes, binoculars, and microscopes. All sorts of things will act like lenses, even things that aren't normally thought of as lenses. As long as they're transparent and curved, they satisfy the definition and will behave in a manner similar to the devices commonly thought of as lenses. Car headlights and taillights are covered with lenses. (They're even called lenses in auto parts catalogs.) A drop of water is a lens. So are fishbowls, light bulbs, and drinking glasses. In general, a lens is any piece of transparent material with at least one curved surface.

Extech’s SP505 measures solar power level from sunlight outdoors to determine the best location of solar panel and solar water heater installations. It is also used indoors by window manufacturers, the automotive industry, and remodeling home owners to check the level of solar radiation transmission through various glass thicknesses and degrees of tinting to use for window installations. The convenient, pocket-sized meter with a built-in precision solar light sensor is easy-to-use wherever you go.

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Insufficient lighting can create security risks or result in OSHA crackdowns. The Extech LT300 light meter can determine whether you have adequate illumination to protect workers, students, visitors, patients or attendees of different venues. This handheld instrument measures up to 40,000 Fc (or 400,000 Lux) utilizing a remote light sensor connected via 12 in. coiled cable (expandable to 24 in.). A prominent LCD readout displays a digital calculation and complementary analog bar graph for quick, reliable assessments. Complete with 9V battery, light sensor with protective cover, protective holster, and soft case.

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This 407026 Heavy Duty Light Meter has 4 lighting type settings for enhanced accuracy (Tungsten/Daylight, Fluorescent, Sodium, Mercury). Additional features include Data Hold, Min/Max/Average, Auto power off and a % reading of the differential from a reference point. Built-in RS-232 interface with optional Data Acquisition Software and Data logger. Complete with 9V battery, light sensor with 45" (1.1m) cable, protective cover, and holster with stand.

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It is often more convenient to work with this equation if we give special names to the inverse quantities. The inverse of focal length is the refracting power (usually just called power) of the lens and the inverse of the object and image distances are called vergences. Using these new terms, the lens equation can be stated more compactly in words and symbols. "The power of a lens is the sum of the object and image vergences." Since it is a linear relationship, it is also much easier to handle mathematically.

In any case, it's only half true that a lens can concentrate sunlight to a point that might then be used to a start a fire. Only a converging lens will do that — a lens that converges parallel rays of light down to a point. Some lenses do just the opposite — they diverge parallel rays of light from a point. Thus lenses are divided into two major categories called converging lenses and diverging lenses, respectively. The point to which these rays converge or from which they appear to diverge is called the focus or focal point and is indicated with the symbol F.

The HD450 Light Meter measures from 0 to 40,000Fc and 0 to 400kLux with a 0.01Fc/0.1Lux resolution. Features: 5% accuracy, Peak mode (10µs), Relative mode, Min/Max, Data Hold, backlit LCD display with 40-segment bar graph, and a rugged double molded housing. In addition, the datalogger function can automatically store up to 16,000 readings or manually store/recall up to 99 readings. Comes complete with light sensor with 3ft (1m) cable and protective cover, software with USB cable, hard carrying case and 9V battery.

The Extech UV505 Pocket Light Meter measures UV-AB light from natural and artificial light sources. It can be used in outdoor activities, indoor medical testing, laboratory and research, industrial, education, electronics, and forensics/criminology applications. It offers a backlit display, cosine corrected sensor, two selectable ranges, zero function, display hold, and auto power off. Includes a pouch to protect and store your meter.

The combination of lenses acts as a single lens. We can locate the focus of this lens combination (F12) by tracing the emergent ray parallel to the principal axis back to the original object. The place where this ray intersects the principal axis is the location of the combined focus.

The real image formed by the first lens serves as a virtual object for the second lens. Since it isn't the original object, it is indicated with the symbol O*. The ray parallel to the principal axis (indicated by a dashed line) will actually strike the second lens before it has a chance to form an image. When it does, it is refracted through the focus of the second lens (F2). The intersection of this ray with the first ray drawn through the center of the two lenses shows the location of the final image formed.

The EA30 Light meter sports a compact and rugged design and features a large display. Multiple wide measuring ranges: 40,000Fc in 4 ranges and 400,000Lux in 5. Data hold freezes reading on display MIN/MAX readings. Zero function Cosine and color corrected measurements. Low battery and overrange indication. Auto power off to save battery life. Meter utilizes precision silicon photodiode sensor with 3ft coiled cable for easy storage. Complete with built-in stand, remote sensor with protective cover, protective rubber holster and 6 AAA batteries.