Visionstar PS-48

The datasheet gives three bands: 0.25 to 4.00 in 0.25 steps, 4.00 to 6.00 in 0.50 steps and 6.00 to 10.00 in 1.00 steps. That is twenty-four values: sixteen from 0.25 to 4.00, then 4.50, 5.00, 5.50 and 6.00, then 7, 8, 9 and 10. The quantity per power is not stated. The model code and the two columns in the photograph suggest two of each, for 48 pieces, but ask the seller for a written packing list.

The datasheet prints the values without a unit. Trial prisms are conventionally graded in prism dioptres, and these values are consistent with that. Confirm the unit in writing, because a prescription written in the wrong unit is a dispensing error that reaches the patient.

The photograph shows a blue fitting with two C-shaped cells on a white handle, stored beside the prisms. It appears to be a hand-held holder for placing lenses in front of both eyes, but the datasheet does not describe it or confirm that it is included. Ask the seller what it is and whether it comes with the set.

The datasheet does not say. A trial prism is only useful if the practitioner knows exactly where its base lies, so the marking method matters. Ask the seller how the base is marked, and check each prism on a focimeter at delivery to confirm that power and base direction match the marking.

It depends on the patient group and the purpose. In the Convergence Insufficiency Treatment Trial, 72 children aged 9 to 17 with symptomatic convergence insufficiency wore base-in prism or placebo reading glasses, and the prism glasses were no more effective than placebo. A 2020 trial of 64 young adults aged 18 to 38 found that base-in prism did reduce symptoms compared with placebo, without changing near point of convergence or fusional vergence. Neither study examined this set. Together they show why a prism should be trialled carefully, and prescribed only where it clearly helps.

Prism needed to relieve a vertical fixation disparity, or an associated phoria measured at near, is often small. Quarter steps up to 4 let the practitioner find the least amount that neutralises the disparity or removes the symptom. Less prism means thinner, lighter spectacle lenses and less for the patient to adapt to.

If each power is supplied twice, as the model code suggests, prism can be divided between the two eyes, which is often preferred for larger amounts. Stacking two prisms in one cell gives only an approximate total, and the error grows with larger values. Confirm the quantity per power before relying on splitting.

The rim outer diameter and thickness are not published, and those decide whether a prism seats firmly in a frame cell. A prism that rotates in its cell changes its base direction, which defeats careful measurement. Ask for both dimensions before order and check them against your frame. BS EN ISO 12867 is the trial frame standard applied in the UK and Europe.

The reference standard is BS EN ISO 9801:2009 for trial case lenses, confirmed by ISO in 2024. The datasheet claims no conformity, so ask for test evidence on prism power tolerances. Under UK MDR 2002 the set is typically Class I, or higher, use-dependent, confirmed against the declaration of conformity at order. The declaration is issued by the manufacturer and provided by the seller. Conformity requirements differ by destination market, and the seller confirms what applies where the unit is going.

In the UK, prism forms part of a prescription issued after a sight test by a registered optometrist or registered medical practitioner under the Opticians Act 1989. Binocular vision assessment and prism trials are also central to orthoptic practice, and orthoptists in the UK are registered with the Health and Care Professions Council. Other countries set their own rules.

The set has no electrical parts, so it sits outside electrical safety and portable appliance testing, and in a UK workplace PUWER 1998 applies as it does to any work equipment. The practical tasks are periodic focimeter checks of power and base direction, and replacing chipped or scratched prisms. Disinfectant compatibility and single-piece replacement pricing are not published, so ask at quotation.

A service measuring large strabismic deviations will find the range ends at 10 per piece. A buyer who needs a documented unit, base marking or holder should get each in writing, since the datasheet gives none. A practice looking for spheres and cylinders will need a separate trial lens set.