WHAT IS lens gauge used for?
Use a lens gauge (clock) Measure front and back curves of lenses. Measure amount of cylinder on lens surfaces.
What is Geneva lens clock?
The Geneva lens clock is used for measuring the base curve of a lens, as seen in the upper image.
How do you find the base curve of a lens?
Figuring out the proper base curve based upon Rx is fairly simple: Plus Power – Use the Spherical Equivalent (Sphere power plus half the cylinder power) and add 4.00 diopter to that. Example – Rx of +2.50, the base curve will be approximately 6.50.
What is Geneva lens measure?
A simple tool to measure the surface curvature of lenses is the Geneva Lens Measure, it was first patented in the US Feb 24,1891. It is a simple 3-point contact gauge, and the basic principle and design is unchanged since its first introduction.
How do you read a lens clock?
When a lens clock is held up against a convex surface, the black numbers indicate the curvature of that surface. A lens clock measuring a +5.00 front/base curvature. When a lens clock is held up against a concave surface, the red numbers indicate the curvature of that surface.
How do you calibrate a lens clock?
Hold the lens clock in one hand & gently depress the pins at the top evenly against the metal surface. With the pins depressed, replace the needle with the other hand on the stem at the zero setting. After setting the needle, test the clock by pulling away & the pressing again against the metal surface.
What base curve should I choose for?
The most basic rule is that you always want the base curve to be as close to +6.00 as you can get and still have the Rx work. In theory +6.00 should always give you the best possible combination of curves for weight, optics, etc.
Is there a big difference between 8.4 and 8.6 base curve?
Studies show that a single base curve of 8.4mm managed a “good or better” fit in approximately 90% of individuals,1 and base curves of 8.4mm and 8.6mm together encompassed 98% of individuals.
How does base curve affect vision?
The base curve of a lens may affect certain aspects of vision, such as distortion and magnification, and wearers may notice perceptual differences between lenses with different base curves. Consequently, some practitioners may specify "match base curves" on a new prescription.
How do you use a Geneva lens clock?
0:241:13Lens Clock Measurer - YouTubeYouTubeStart of suggested clipEnd of suggested clipThe outer two are stationary. And the center contact point moves in and out a lens measure or lensMoreThe outer two are stationary. And the center contact point moves in and out a lens measure or lens clock is held perpendicular to the lens surface. With the center contact point at the optical.
What is a lens caliper?
This Lens Thickness Caliper precisely measures the thickness of lenses up to 19 mm. Throat has a maximum depth of 3.8 mm and is calibrated in 0.20 mm increments. The Lens Thickness Caliper is a stainless steel construction with a brass index plate.
How do you use a lens thickness caliper?
0:001:042060 2060WM Lens Thickness Caliper - YouTubeYouTubeStart of suggested clipEnd of suggested clipAnd II that readout is simple and straightforward with tenths of a millimeter calibration. TheMoreAnd II that readout is simple and straightforward with tenths of a millimeter calibration. The widemouth lens caliper opens to a maximum of 25 millimeters. It's usually used in a surfacing.
How do you use a Geneva lens clock?
0:241:13Lens Clock Measurer - YouTubeYouTubeStart of suggested clipEnd of suggested clipThe outer two are stationary. And the center contact point moves in and out a lens measure or lensMoreThe outer two are stationary. And the center contact point moves in and out a lens measure or lens clock is held perpendicular to the lens surface. With the center contact point at the optical.
What is the Geneva standard index?
the standard index of refraction is 1.53. Also may be called the Geneva standard, it is used for calibration of optical instruments. the weight in grams of one cubic centimeter of any given material. The higher the Specific Gravity of the material from which a lens is made, the more it will weigh.
Why is Pantoscopic tilt important?
Pantoscopic tilt is most always recommended since it helps achieve a close fit by balancing the vertex in the 90 degree meridian. In addition, proper pantoscopic tilt will help maximize the amount of bridge surface resting on the nose.
What are meniscus lenses?
Meniscus lens is a lens which has two spherical curved surfaces, convex on one side and concave on the other side. It is thicker at the center than at the edges. The lens provides a smaller beam diameter in order to reduce the beam waste and spherical aberration.
What is a lens clock?
A lens clock is a mechanical dial caliper measuring dioptric power of a lens curvature. It is a specialized version of a spherometer.
What is the best way to measure contact lens thickness?
Hard and gas permeable (GP) contact lens thi cknesses are best measured with a contact lens dial thickness gauge. However, since most ophthalmic practices do not have this tool, but they do have a lens clock, the lens clock can be used to measure the approximate thickness of the contact lens.
How to measure RGP contact lens?
This is how to perform this measurement with a lens clock, as described on the Art Optical web site: Place the contact lens, concave side up, on a clean, flat, hard surface, such as a desk or countertop. Place the center prong of the lens clock in the absolute center of the contact lens, this will leave the other two prongs on the flat surface on the outside of the contact lens. Take the reading off the dial of the lens clock and divide by two. This will approximate the thickness of the RGP contact lens.
How to find the center thickness of a contact lens?
Example: Lens clock reads +2.50 diopters, divide by 2 = 1.25, move the decimal point one place to the left = .125, this will equal the approximate center thickness of the contact lens in millimeters.
What is the calibrated index of Royal Crown Glass?
Remember the dial read -3.0 for the first side, but is calibrated for an index of Royal Crown Glass (1.523).
lens clock
Can one of you please talk me through using a lens clock? I am a little confused about it. There was a really good article in visioncareproducts.com on how to use it, but I have never actually used one and wanted to be sure I was doing it right. I appreciate all advice!! Please help the practical is June 28 I have got to be prepared.
The Good old Geneva!
The good old lens clock basically works on saggital depths (I have probably mis-spelt that) or sags. The face on the older ones actually read sags which you then had to work the power from. This may sound silly, but at the time they were mainly used for two different indices (Pre CR39) 1.523- Crown and 1.530- standard tooling.
What is a lens clock?
Answer: A device used to determine the base curve of the back surface of a spectacle lens. It is often used clinically to detect plus cylinder spectacle lenses in an individual who is use to minus cylinder lenses. It is specifically calibrated for the refractive index of crown glass (n = 1.53). Special lens clocks are available for plastic lenses.
What does the lens clock measure?
The lens clock physically measures the sagital height/depth.
How does a lens measure work?
This device physically measures the sagital depth of a refracting surface and calculates the refracting power of the surface. A pointer that is activated by a system of gears indicates the position of the movable pin in relation to the fixed pins. If the instrument is placed on a flat surface, the protrusion of the central pin is equal to that of the fixed pins, with the result that the scale reading is zero. If placed on a convex surface, the protrusion of the central pin is less than that of the fixed pins, but if placed on a concave surface, the protrusion of the central pin is greater. Because the chord length (the distance between the two outer pins) has a constant value for the instrument, the position of the central pin, indicates the sagitta of the surface, which provides a direct reading of diopters of refracting power of a surface of the lens.
What eye do you use for retinoscope?
Normally, the examiner will use their right eye to perform retinoscopy on the patient's right eye and their left eye for the patient's left eye. The examiner should align themselves just off-center to minimize lens reflections and to allow the patient to visualize the distance target to relax their accommodation. The patient should be instructed to look at a distance target such as a large Snellen letter (20/200-20/400).
How to calculate lenticular astigmatism?
Answer: Take the amount of with the rule astigmatism noted by keratometry readings, multiply that by 1.25, and then subtract that number from 0.75 diopters (lenticular astigmatism).
How does a retinoscope work?
Most retinoscope today use a streak projection system. This streak of light is reflected from a mirror. Additionally, the streak can be moved in relation to a convex lens in the device by way of the sleeve. This allows the light to leave the device as if it were coming from a point behind the retinoscope ( plano mirror setting) or as if it were coming from a point between the examiner and the patient (concave mirror setting). For Copeland retinoscopes, the plano position is with the sleeve up, while the Welch Allyn retinoscope is in the plano position with the sleeve down.
What happens to the magnification of a condensing lens?
As the power of the condensing lens decreases, the magnification increases. Axial magnification increases exponentially, based on the formula Axial magnification: M A = (M) 2.
Lens clock
A lens clock is a mechanical dial indicator that is used to measure the dioptric power of a lens. It is a specialized version of a spherometer. A lens clock measures the curvature of a surface, but gives the result as an optical power in diopters, assuming the lens is made of a material with a particular refractive index.
How it works
The lens clock has three pointed probes that make contact with the surface of the lens. The outer two probes are fixed while the center one moves, retracting as the instrument is pressed down on the lens's surface. As the probe retracts, the hand on the face of the dial turns by an amount proportional to the distance.
Radius of curvature
The radius of curvature R {\displaystyle R} of the surface can be obtained from the optical power given by the lens clock using the formula
Estimating thickness
A lens clock can also be used to estimate the thickness of thin objects, such as a hard or gas-permeable contact lens. Ideally, a contact lens dial thickness gauge would be used for this, but a lens clock can be used if a dial thickness gauge is not available. To do this, the contact lens is placed concave side up on a table or other hard surface.
