A question

[Gavin_Aus]

I'm quite new to working as an optical dispenser and have a question regarding frame adjustments...

What is meant by...

'Let-Back' & 'Front Angle'

Also i'm needing to know the main differences between the datum & boxing system used for frame measurements.

Any help would great.

Cheers,
Gav.

[Texas Ranger]

Suppose they're Aussie terms, although "front angle" could be pantoscopic tilt?

[Laurie]

Hello and Welcome,

The Datum system was orginally used to measure the A, B, and other frame dimensions of frames. The only problem was, it did not include all of the "boxed lines" as the boxing system does, which left room for error in the DBL, necessary for accurate OC placement for PD's.

Today we use the boxing system...same concept, but more accurate. With the boxing system, you "box out" the eyewires, and add lines horizontally and vertically that surround each eyewear at perpendicular angles.

This allows more accurate measurements of frames (especially DBL) and is used world-wide. We know that a frame manufacturer uses the boxing system when they include a small box between the A and DBL numbers.

Hope this helps,

Laurie

[David Wilson]

Hi Gavin,
As Laurie says, we now use the boxing system. The datum system uses the same top and bottom tangents to the lens (the top and bottom of the box) but eyesize and bridge (DBL) are made on the datum line, what the boxing system calls the horizontal centreline. Confusing the two systems has led to many errors in centration. Frontal angle is not the same as pantoscopic tilt. It is the angle of the lens shape at the bridge when viewed from the front - the nasal shape, if you like. Let back is the angle made by the temples when opened and viewed from above. This is usually slightly over 90 degrees.

These terms are not just Aussie terms. They are found in several international puplications. I haven't got a copy of Brooks' 'System for Ophthalmic Dispensing handy at the moment but I wouldn't mind betting that the terms are in there.
Regards
David

[Darryl Meister]

Yeah, those terms David described are used in the US as well, though most dispensers probably haven't been exposed to them. Also, both let-back angle and set-back angle are used to describe temple spread over here. System for Ophthalmic Dispensing (the stand text on the subject), for instance, uses "let-back" angle while the Z80.5 standard uses "set-back" angle.

Best regards,
Darryl