Tips for selecting a useful stand magnifier

Transcription

1 Tips for selecting a useful stand magnifier William L. Brown, O.D., Ph.D., F.A.A.O. Mayo Clinic Rochester, MN Ph: (507) brown.william2@mayo.edu

2 Learning Objectives Following the lecture the participant will be able to: Understand the optical design of stand magnifiers Predict how the use of a bifocal add will affect the equivalent power of the stand magnifier system Select a stand magnifier that is appropriate for the patient s magnification needs. Understand the misleading characteristics of magnification labeled on stand magnifiers

3 Stable image Good illumination (illuminated SM) Incandescent Halogen LED Bright, cold light Very energy efficient Great battery life Advantages of SM

4 Disadvantages of One hand occupied Bulk and weight Material must be well supported Fixed focus Reading distance critical Challenges for illumination when non-illuminated Reflections when noniluminated External light not needed if self-illuminated SM

5 Magnification, lens size, field of view and reading distance Len size as lens refracting power (& magnification) Field of view as either Lens size OR Magnification UNLESS Reading distance Generally Higher powered SMs are designed to be held closer to the spectacle plane

6 Image location Add power What determines where the SM should be held? Intention and design of the manufacturer

7 Important descriptors Entering vergence, L F m Emerging vergence, L y Image y Object Magnifier refracting power, F m Emerging vergence L (=L + F m ) Image location l = 1/L Need for add Maximum allowable add = -L l Magnifier Emmetrope or corrected ametrope Enlargement ratio m = Image ht / object ht = y / y Same as lateral or transverse magnification

8 Example of descriptors Eschenbach Specs taken from Eschenbach online description: om/products/standilluminated-systemvarioplus-head asp?catid=b45f491dbb9 C4F67AD455325BF226C67 Specs: (WLB comments in red) Magnification: 5x Diopter: 20D Lens Type: Aspheric Lens-to-Image Distance: 5.90 in. (=15 cm) L = 40/-5.9= -6.8D Max Allow Add = -L = +6.8 D Transverse Magnification/Enlargement: 3.1x Max. Allow. Add: 6.7D Lens Size: 3.14 in.

9 Meaning of Add (F A ) Power in spectacle plane over and above the lens power necessary to correct refractive error Accommodation Add power Under- or over-corrected refractive error Total add F A = Acc + Add + Refr error + Dist Rx Myopia (+) Hyperopia (-)

10 For example Assume the total add is Could be an emmetrope using a D add OR 3.00 D myope wearing 2.50 DS / 1.50 D add accommodates 0.50 D while using a stand magnifier. What is the total add used to view the image? F A = Acc + Add + Refr error + Dist Rx = (-2.50) = D Total Add (undercorrected myopia = +0.50)

11 An example How far from the spectacle plane must this SM be held if: 1) the image is 15 cm from the SM and 2) it is used with a 2.50 D add? 2.1 X larger than the original object: Enlargement ratio Stand Magnifier F m = +7.1D Total Add = D (to focus image at 40 cm) Image seen thru mag (Focal plane of D add) l' = 15 Object t =? Viewing Distance cm Image distance required for 2.50 D add = 40 cm t = = 25 cm Emmetrope or corrected ametrope

12 How far from the spectacle plane must the SM be placed for the image to be seen clearly? Image is 15 cm from magnifier. Focal plane of D total add is 100/(2.5D) = 40 cm from the spectacle plane. Image from magnifier must be 40 cm from spectacle plane. Therefore, the distance from the spectacle plane to the magnifier must be: t = 40 cm 15 cm = 25 cm

13 Magnifier data* (Sample data below for illuminated SM) Measured Manufacturer ID # Description Lens size P e v ER mm D. cm x COIL x-20D Raylite COIL x-15.6D Raylite Eschenbach X/7.6D Eschenbach X/7D Lighthouse 4X 4xPower M ag Lighthouse 2.8x 2.8xPower M ag Schweizer 162 8X Schweizer Okulux 28D Schweizer 148 8x Leuchtlupe *From Berkeley Low Vision Yellow Pages

14 COIL Raylite SM

15 Focus hand held telescope onto a very distant object (to minimize accommodation) Place plus trial lenses, starting with too much plus, on the upper surface of SM Find trial lens for which image is clear. Emerging vergence is negative of trial lens. Image distance is 1/(trial lens power). If tables not available Measure the image distance, l

16 Example Trial lens needed to see image clearly is D on SM. Image distance = 1/ (-6.50D) = -15 cm

17 Which magnifier should I use? Begin with the add needed to read the goal print: Newsprint For example: 5 D add needed to read newsprint This add becomes the equivalent power needed in any device to read newsprint.

18 Equivalent power*: Enables comparison of devices Devices with the same equivalent power produce the same retinal image size. * Alternative method is equivalent viewing distance: EVD = 1/F eq

19 What is the equivalent power of a magnifier? Equivalent power of the magnifier by itself vs (Given in the tables) Equivalent power of the magnifier when used with an add as a system How do I know the equivalent power of the magnifier/add system?

20 Let s go back to our example 2.1 X larger than the original object: Enlargement ratio Stand Magnifier F m = +7.1D Total Add = D (to focus image at 40 cm) Image seen thru mag (Focal plane of D add) Object t = 25 cm Viewing Distance l' = 15 cm Image distance = cm = 40 cm E Emmetrope or corrected ametrope

21 How do I calculate F eq? Simply use the enlargement ratio and total add ER = Image size / object size ER = 2.1 (from tables), Add = 2.5 D F eq = (ER) x Add F eq = (2.1) x (2.5 D) = +5.2 D (Easier than equivalent power equation) F eq = F m + F A t F m F A = (+7.1D) + (2.50) (.25m)(7.1)(2.5) = 9.6 D 4.4 D = 5.2 D

22 Remember, our patient could read newsprint with a 5 D add? Therefore any system (magnifier and add) that has an F eq of 5 D or larger should allow newsprint to be read. With our 7.6 D SM and a 2.5 D add: F eq = +5.2 D our patient should be able to read newsprint.

23 Selecting the SM: Easy as 1, 2, 3, 1. Determine required equivalent power for goal Find spectacle add needed to achieve goal E.g., 20 D add needed to read newsprint, but working distance too short. 20 D becomes the required equivalent power. 2. Choose add to be used with SM Assume pt wears 3.00 D add If suitable SM can t be found with following steps, different add may be necessary. 3. Calculate ER ratio needed to achieve goal using the chosen add ER = F eq / Add = 20/3 = 6.7

24 Look in tables for a magnifier with ER = 6.7 or slightly larger (Or better yet, LABEL your SMs with ER and image distance!) Measured Manufacturer ID # Description Lens size P e v ER mm D. cm x COIL 6289 Raylite S eries Schweizer Okulux 28D COIL x-28D Raylite Lighthouse 7X 7xPower Mag Schweizer 148 8x Leuchtlupe Eschenbach X/23D COIL x-44D Hi-power All viewing distances work with a add ( 33 cm)

25 Examples of illuminated SM Examples: with ER 6.7 Magnifier ER F eq (w/ 3.00 add) COIL 5289, 8X D Schweizer PowerMag7X D Esch X D COIL X D

26 THAT S IT! Hope this helps sort out some of the confusion about stand magnifiers However, it really gets confusing if you try to use magnification labeled on the magnifiers

27 DON T try to use traditional relative magnification! The magnification labeled on the magnifier usually cannot be achieved!! 1 What does it mean to you if you see a magnifier labeled 3X? 1 Brown WL, Siemsen DW. Magnification labels for stand magnifiers: always misleading and usually unachievable. Optometry 2008 Jan; 79(1):9-17.

28 Relative Magnification: Definition RIS c magnifier RIS at reference distance s magnifier (RIS = retinal image size) d O 25 cm

29 Special Case 1: Assumptions Object is at the focal plane of the magnifier Emerging vergence = 0 Therefore total add used with magnifier = 0 Reference distance = 25 cm Then M r = F m / 4 Rated Magnification F m ag f m ag F m E m e rg ing vergence = 0

30 Rated Magnification: Our 3X example If rated magnification used to label: F m = 4 M r = (4) (3) = +12 D Focal length of magnifier = 1 / 12 = 8.3 cm 3X then means: patient holds the page at the focal plane of the magnifier, 8.3 cm from the magnifier in this case retinal image with the magnifier is 3X larger than the retinal image when the magnifier is removed and object is viewed at the reference distance, 25 cm. 4 D of accommodation must be used to see the object located 25 cm away. (That s basically the 4 in the equation.)

31 Rated magnification & stand magnifiers For stand magnifiers with a negative emerging vergence, rated magnification is not applicable. WHY NOT? Entering vergence, L F m Emerging vergence, L Image Im a g e Object O b je c t Magnifier Emmetrope or corrected ametrope

32 Question: Can the patient place the page at the focal plane of the SM? Answer: Yes, by raising the magnifier off the page Brings SM focal point closer to the page Sends image farther from SM, closer to infinity Decreases divergence entering spectacle plane If a patient tells you it is clearer when the SM is lifted off the page, it tells you the add is not strong enough (or they are not using the add )

33 Special Case 2 Conventional Magnification Assumptions Magnifier held at spectacle plane Object is inside focal plane of magnifier, so image is at the reference distance of 25 cm Emerging vergence = -4.00D Therefore total add used with magnifier =+4.00D Reference distance = 25 cm Then M r = F m / 4 + 1

34 Conventional Mag: Our 3X example If conventional magnification used to label: F m = 4 (M r - 1) = (4) (3-1) = + 8 D Focal length of magnifier = 1 / 8 = 12.5 cm 3X then means: 1) patient holds the magnifier close to the eye, and 2) moves the page to the proper distance (closer than 12.5 cm ) so that the image is at 25 cm retinal image with the magnifier is then 3X larger than the retinal image when the magnifier is removed and object is viewed at the reference distance, 25 cm of total add is required to view the image. In this case the 8 D magnifier is getting credit for an extra 4.00 D of power, the power from the total add

35 What about the SM in our first example? Recall: F m = +7.1 D Image is 15 cm from magnifier Rated mag = (7.1 / 4) = 1.8X Assumes that page is at focal plane Doesn t apply Conventional mag = (7.1 / 4) +1 = 2.8 X Applies only if the magnifier is held close to the eye and the image is viewed at 25 cm But with magnifier close to eye, image is 15 cm, not 25 cm from eye Conventional mag doesn t apply either, even though it is used to label.

36 Useful fields of view in stand magnifiers Total field of view Resolvable field of view Optimized viewing distance (especially for high powered SM)

37 Resolvable Field of View: 20-24D Horizontal FOV (mm) 25 Schweizer D 52mm PowerMag D 46mm Eschenbach D 54mm COIL D 35mm 15 Mag to image dist (ER) Eye to SM distance (cm) Schw 16.7cm (4.2X) PM 28 cm (6.9X) Esch 22.1cm (4.9X) COIL 24.4cm (6.9X)

38 55 45 Total Field of View: Field of View: 20-24D 20-24D 35 Horizontal FOV (mm) 25 Schweizer D 52mm PowerMag D 46mm Eschenbach D 54mm COIL D 35mm Eye to SM distance (cm)

39 Resolvable FOV Varies significantly among SMs and is an important consideration when prescribing. Usefulness of a SM cannot be predicted solely by the equivalent power of the system. Not all SMs are created equal for all working distances. At certain working distances the extent of the useful image is quite limited.

40 Bar magnifiers Magnification in one direction only. Magnification = index of refraction of magnifier Index for all magnifiers close to 1.5, so magnification is approximately 1.5X Despite the physical size of the magnifier, magnifications are virtually the same. Plane of image close to object plane Easy to focus back and forth between magnified and unmagnified images.

41 Bar magnifiers Potentially useful for patient with island of vision Spherical magnification pushes magnified image out of island of vision. Bar magnifier maintains same visual span horizontally while magnifying vertically.

42 Object plane is at center of curvature, C, of the cylinder Therefore image plane is the same as the object plane n = 1.0 n = 1.5 General assumptions about cylindrical (bar) magnifiers C, object plane, & image plane, but image 50% larger than object If we assume: n = 1.5 = index of refraction of plastic magnifier n = 1 (for air surrounding the magnifier) Object plane is at center of curvature of cylinder: L' L F n' n n' n If n 1.5 and n' 1, then l' l r l' r r r 1 1 l' r l' r and nl' m n' l 1.5r r 1.5

43 51 yo CF, long history of Stargardt s CC: Trouble reading large print, using combination of reading glasses & illuminated Ott Light Magnifier Would like stronger reading glasses to replace this combination too cumbersome and print difficult to read Content to continue large print (2M). VA with current bifocals RT 5 ft/125 LT 5 ft/40 Refraction no change Eccentric viewing both eyes Near only glasses LT (power = add) 0.23/2.0 MNRD

44 Original visual span 8 letters Original visual span, no magnifier = 8 letters Magnify 1.5X overall cuts visual span to 5 letters Magnify 1.5X vertically only maintains 8 letter visual span

45 Thank you!