What Is an Optical Scale (Shkala) in Precision Optics and How Is It Used?

An optical scale (Russian: шкала, shkala) is a graduated measuring element—a substrate carrying precisely spaced marks, lines, or numbers—that is read visually or optoelectronically inside an optical-mechanical instrument. It is used wherever an instrument must report a position, angle, or displacement as a number: measuring microscopes, theodolites, levels, spectrometers, comparator benches, and machine-tool reading heads. You choose a scale when the requirement is a quantitative readout; you choose a reticle (сетка, setka) when the requirement is aiming, centering, or alignment against a reference pattern. Both are precision optical components, and NPP «Fokus +» lists шкала and сетка as separate product categories alongside lenses, prisms, mirrors, protective glass, optical windows, and filters—a useful signal that the two are specified and manufactured as distinct items.

What an optical scale actually does

A scale converts a physical quantity into a readable graduation. The substrate is transparent (glass) or reflective (metal, glass with a reflective coating), and the graduations are produced by a marking method suited to the required accuracy. The instrument's optics image the scale onto an eyepiece, a projection screen, or a detector; the operator or the electronics then interpolates between marks.

Three functions recur across instruments:

• Measurement — the scale is the metrological reference. A measuring microscope's stage position, a spectrometer's wavelength drum, or a level's tilt is read off the graduations.
• Alignment — a scale with a zero mark or a coarse/fine pair lets an operator return a mechanism to a known position before fine adjustment.
• Readout — in optical-mechanical devices the scale is the human-machine interface; in modern versions the same graduations are imaged onto a photodetector array for digital output.

Scale vs. reticle: how to choose

The two components look similar—both are patterned optical plates—but they answer different questions.

In practice many instruments use both: a reticle to aim at the feature and a scale to read how far the stage moved. If your requirement is "tell me the number," specify a scale; if it is "put this on the crosshair," specify a reticle.

Specification factors that matter

When writing a requirement for a scale, the following determine whether the part will work in the assembly:

• Substrate material and finish — optical glass for transmitted-light readout, or a reflective substrate where the scale is read by reflected light. Surface quality and flatness must match the imaging optics; a wavy substrate blurs graduations across the field.
• Graduation accuracy and spacing — the tolerance on line position, not just line width. This is the parameter that sets the instrument's measurement uncertainty, so state it as a positional tolerance over the full scale length.
• Marking method — the manufacturing route (photolithographic patterning, etching, mechanical ruling, or deposition) must be compatible with the required line width and accuracy; the finer the graduations, the more the method constrains the design.
• Line width and contrast — narrow lines improve resolution but reduce contrast and demand better illumination; the two are traded against each other.
• Numbering and orientation — font size, placement, and reading direction must suit the eyepiece or projection optics, including any image inversion in the system.
• Overall dimensions and mounting — thickness, edge finish, and how the plate seats in its holder; a scale that rocks in its mount loses accuracy regardless of graduation quality.

Integration and inspection

A scale is only as good as its mounting. In a precision optical assembly, check that the scale sits in a reference seat with no play, that its graduations lie in the intended image plane (or are imaged at the intended magnification), and that illumination is even across the full graduation length—vignetting at the ends is a common cause of readout error that looks like a graduation defect.

For incoming or final inspection, verify:

1. Graduation spacing against the drawing tolerance, sampled at several positions along the scale, not only at the center.
2. Line quality — breaks, bridges, or thickness variation under magnification.
3. Numbering legibility at the actual readout magnification, not just under a bench microscope.
4. Substrate defects — scratches, bubbles, or coating voids in the clear aperture.
5. Seating and orientation in the instrument, confirmed by reading a known reference position.

If a scale fails, the cause is often the mount or the illumination rather than the pattern itself, so inspect the installed assembly before rejecting the component.

Practical takeaway

Specify an optical scale when the instrument must report a measured value, and a reticle when it must aim or align; state graduation positional accuracy, substrate, marking method, and mounting interface as the four controlling parameters. NPP «Fokus +» manufactures scales and reticles as part of its precision-optics range, so both can be sourced against a single drawing set—but confirm the graduation tolerance and inspection method in the drawing, since those, not the catalog category, determine whether the part meets the instrument's measurement budget.