physics

Optical comparator

Optical comparator is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Optical comparator rather than just read about it. In short: An optical comparator (often called just a comparator in context) or profile projector is a device that applies the principles of optics to the inspection of manufactured parts. In a comparator, the magnified silhouette of a part is projected upon the screen, and the dimensions and geometry of the part are measured against prescribed limits.

Key takeaways

Optical comparator belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Optical comparator to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Optical comparator from memory before moving on to harder problems.

Reference excerpt

An optical comparator (often called just a comparator in context) or profile projector is a device that applies the principles of optics to the inspection of manufactured parts. In a comparator, the magnified silhouette of a part is projected upon the screen, and the dimensions and geometry of the part are measured against prescribed limits. It is a useful item in a small parts machine shop or production line for the quality control inspection team. The measuring happens in any of several ways. The simplest way is that graduations on the screen, being superimposed over the silhouette, allow the viewer to measure, as if a clear ruler were laid over the image. Another way is that various points on the silhouette are lined up with the reticle at the centerpoint of the screen, one after another, by moving the stage on which the part sits, and a digital read out reports how far the stage moved to reach those points. Finally, the most technologically advanced methods involve software that analyzes the image and reports measurements. The first two methods are the most common; the third is newer and not as widespread, but its adoption is ongoing in the digital era.

History The first commercial comparator was developed by James Hartness and Russell W. Porter. Hartness' long-continuing work as the Chairman of the U.S.'s National Screw-Thread Commission led him to apply his familiarity with optics (from his avocations of astronomy and telescope-building) to the problem of screw thread inspection. The Hartness Screw-Thread Comparator was for many years a profitable product for the Jones and Lamson Machine Company, of which he was president. In subsequent decades optical comparators have been made by many companies and have been applied to the inspection of many kinds of parts. Today they may be found in many machine shops. The idea of mixing optics and measurement, and the use of the term comparator for metrological equipment, had existed in other forms prior to Hartness's work; but they had remained in realms of pure science (such as telescopy and microscopy) and highly specialized applied science (such as comparing master measuring standards). Hartness's comparator, intended for the routine inspection of machined parts, was a natural next step in the era during which applied science became widely integrated into industrial production.

Usage The profile projector is widely used for complex-shape stampings, gears, cams, threads and comparing the measured contour model. The profile projector is hence widely used in precision machinery manufacturing, including the aerospace industry, watches and clocks, electronics, the instrumentation industry, research institutes and detection metering stations at all levels, etc.

Work principle

The projector magnifies the profile of the specimen, and displays this on the built-in projection screen. On this screen there is typically a grid that can be rotated 360 degrees so the X-Y axis of the screen can be aligned with a straight edge of the machined part to examine or measure. This projection screen displays the profile of the specimen and is magnified for better ease of calculating linear measurements. An edge of the specimen to examine may be lined up with the grid on the screen. From there, simple measurements may be taken for distances to other points. This is being done on a magnified profile of the specimen. It can be simpler as well as reduce errors by measuring on the magnified projection screen of a profile projector. The typical method for lighting is by diascopic illumination, which is lighting from behind. This type of lighting is also called transmitted illumination when the specimen is translucent and light can pass through it. If the specimen is opaque, then the light will not go through it, but will form a profile of the specimen. Measuring of the sample can be done on the projection screen. A profile projector may also have episcopic illumination (which is light shining from above). This is useful in displaying bores or internal areas that may need to be measured.

Features

Projection methods Vertical projector: The main axis is parallel to the plane of the screen. They're most common, and suitable for flat parts or smaller work-pieces. Horizontal projector: The main axis is perpendicular to the plane of the projection screen. Screens are thus made mainly in medium and large versions generally suited for examining shaft parts or heavy work-pieces with large profiles, although having a horizontal table below without a hole for light transmission can be convenient for small machines with a silhouette lighting arrangement.

Positive or inverted images For the simplest type of profile projector, the part's inverted image, also known as its mirror image, will be displayed on the screen. In order to facilitate the measurement, sometimes a plus-image system is deliberately added, changing the inverted image into a positive one, which increases the cost due to scale/material used, while somewhat reducing its measurement accuracy.

Screen size As for selection of screen size, one should carefully consider whether the entire part must be imaged on the screen. If the inspection can readily be done at a modest scale, there is no need for a larger screen. Projector manufacturers offer multiple screen sizes to meet various needs.

Magnification The projection lens magnification is fixed. Different views of measured pieces often require different magnifications. However, the usual projector factory configuration is with a single lens, so according to needs, additional lenses may be purchased and used.

Work table and accessories The work table is used to place and hold the measured piece. Its own volume, X, Y travel and carrying capacity are critical. Meanwhile, for the convenience of holding the workpiece, a precision rotary table, a V-block part holder and other accessories are generally added. Also, the projector must have a flexible and stable focusing mechanism and large working distance (the top surface of the workpiece relative to the lens pitch). The user selects appropriate data processing modes: without exception, all modern optical measuring projectors on market have been digitized.

Illustrations

Profile projector, also known as contour comparator, is widely used to measure 2-dimensional data.

Profile projector working structure

Worked examples

Example 1 — a first encounter with Optical comparator

Start with the simplest possible case. Write down what Optical comparator claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Optical comparator before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Optical comparator ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Optical comparator

In research: Optical comparator appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Optical comparator in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Optical comparator is common in secondary-school and first-year university syllabi. It links to neighbouring topics Industrial equipment, Metalworking measuring instruments, Metrology, so understanding it makes those chapters shorter.
In everyday life: Look for Optical comparator outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Optical comparator in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Optical comparator means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Optical comparator out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Optical comparator in simple terms?

Why does Optical comparator matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Optical comparator?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Optical comparator.