After working through this chapter, you should be able to:
Explain what focimetry is
Discuss the principles underlying focimetry
Be able to link focimetry with your understanding of spherical and cylindrical refractive errors (see chapter 5 )
In this chapter we’re going to think about lenses and how we can measure the power of lenses using an instrument called a focimeter (internationally it is also known as a lensometer – but they are the same thing). This chapter will be most useful to you if you have access to a focimeter to try out the theory and have a go, but it will still (hopefully) be interesting even if you can’t apply it practically.
What is a focimeter?
A focimeter is a device that can be used to determine the spherical power, cylindrical power (and corresponding axis), prismatic power and the optical centre of a lens. You may be thinking, why on Earth would we need a device that does that? But in practice this is an important instrument which allows clinicians to measure the power of a patient’s glasses, even if the patient themselves doesn’t know their prescription. It also allows dispensing opticians to check the power of lenses before dispensing them to patients.
In broad terms, there are three main types of focimeter:
Conventional (eyepiece focusing)
Projection (screen focused)
Automatic electronic (automated)
In this chapter we will focus on the first type – the conventional focimeter – but it’s good to be aware that other types exist.
How does a focimeter work?
The focimeter itself requires two main parts – a focusing system (collimator, which narrows the beam of light) and an observation system (telescope, which focuses the image for the user); see Fig. 11.1 for a schematic of this. The focusing system its elf aims t o produce an image of the target (illuminated lines) overlaid onto a graticule (a pattern of lines in an optical device used as a measuring scale; sometimes called a reticle) and project it to infinity. As the focimeter is used to measure a large range of lens powers, logic tells us that the target (acting as the object) will need to be moveable in order to position the target (acting as an object) at the correct location so as to project it to infinity. However, for the focimeter to work correctly, the eyepiece of the observation system also needs to be focused for your eyes (to account for any uncorrected prescription you might have) before you introduce the lens you’re trying to measure. This will mean that the target and graticule are all lovely and in focus ( Fig. 11.2 ).
Assuming that the eyepiece of the focimeter is focused appropriately for the user, the user can place a patient’s spectacle lens onto the lens (frame) rest ( Fig. 11.3 ). Importantly, the back surface of the lens shoul d be sitting on the lens rest and will need to be clamped into position in order to provide an accurate reading. However, it’s also important to note that before clamping the lens, it will need to be moved upwards/downwards and leftwards/rightwards until the target is visibly aligned with the centre of the graticule. At this point, the user will have located the optical centre of the lens, and most focimeters have a process for allowing the user to mark this point on the lens (to help later on). It’s important to find the optical centre, because this is the point where no prismatic effect will be introduced (see chapter 9 for review on this) and so the power will be able to be accurately determined.