Calculation of EVD and EVP
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This document discusses the calculation of equivalent viewing distance (EVD) and equivalent viewing power (EVP) for low vision patients. It provides the following key points: EVD is the distance at which an object would need to be viewed to achieve the same visual angle as the magnified image. It is calculated as the actual viewing distance divided by the enlargement ratio. Various optical devices that can provide magnification are described, including spectacle magnifiers, handheld magnifiers, stand magnifiers, telescopes, and video magnifiers. The EVD for each device depends on factors like the magnifying power and the distance between lenses or from the lens to the image. Examples are provided to demonstrate calculating the
Calculation of EVD and EVP
- 1. Calculation ofCalculation of EVD & EVPEVD & EVP Rabindra Adhikary ravinems@iom.edu.np
- 2. Equivalent Viewing Distance (EVD)Equivalent Viewing Distance (EVD) Defn : ◦ It is the distance at which the object itself would subtend an angle that is equal to the angle that is being subtended by the image. ◦ Actual working distance from the spectacle plane
- 3. Calculation of EVD for the patientCalculation of EVD for the patient EVD = Eye to Image distance Enlargement ratio ◦ Eye to image distance = Viewing distance ◦ Enlargement ratio (ER) = Best near Acuity (BNA) Target near Acuity (TNA) Equivalent viewing power = 1/ EVD (m)
- 4. Method of calculation of EVDMethod of calculation of EVD Specify resolution at near by M notation ◦ Record print size & viewing distance MeterMeter EquivalentEquivalent SnellenSnellen EquivalentEquivalent Size of TextSize of Text 0.4 M0.4 M 20/2020/20 0.5 M0.5 M 20/2520/25 Footnotes, Phone books,Footnotes, Phone books, 0.8 M (N0.8 M (N66)) 20/4020/40 Paperback PrintPaperback Print 1.0 M (N1.0 M (N88)) 20/5020/50 Newspaper PrintNewspaper Print 1.2 M (N1.2 M (N1010)) 20/6020/60 Magazine PrintMagazine Print 1.6 M1.6 M 20/8020/80 Children’s BookChildren’s Book 2.0 M2.0 M 20/10020/100 Large Print MaterialLarge Print Material 4.0 M4.0 M 20/20020/200 Newspaper subtitleNewspaper subtitle 5.0 M5.0 M 20/25020/250 Newspaper headlineNewspaper headline
- 5. Method of calculation of EVDMethod of calculation of EVD Step one ◦ Take note of the smallest print read with acceptable accuracy & Viewing distance ◦ Take note of accommodation or reading add ◦ Predict the target acuity (Resolution goal) : Calculate ER ? Step two ◦ Predict the distance required to meet the resolution goal ◦ Calculation = Viewing distance for BNA/ER = EVD
- 6. Method of calculation of EVDMethod of calculation of EVD Step three ◦ Verify the predicted EVD that allows resolution goal ◦ Predict EVD (app. Add) at spectacle plane that gives clear focus Step four ◦ Consider the optical system that provides same EVD (EVP) Spectacle magnifier, Hand Held magnifier, Stand magnifier, Telemicroscope, Electronic Devices
- 7. Method of calculation of EVDMethod of calculation of EVD Example: Target acuity 0.8 M (N6) Patient 1Patient 1 (Hari)(Hari) Patient 2Patient 2 (Ram Bdr.)(Ram Bdr.) AgeAge 20 yrs20 yrs 70 yrs70 yrs Smallest printSmallest print readread 2.0 M (N2.0 M (N1616)) 4.0 M (N4.0 M (N3232)) Viewing distanceViewing distance 12 cm12 cm 32 cm32 cm AddAdd NoneNone 2.50 Ds2.50 Ds Acc. DemandAcc. Demand 8.0 Ds8.0 Ds 0.50 Ds0.50 Ds Enlargement ratioEnlargement ratio 2.0/0.8 = 2.5 x2.0/0.8 = 2.5 x 4.0/0.8 = 5x4.0/0.8 = 5x EVD (required)EVD (required) 12/2.5 = 5 cm12/2.5 = 5 cm 32/5 = 6.3 cm32/5 = 6.3 cm EVP = 1/EVDEVP = 1/EVD 1/5 cm = +20 Ds1/5 cm = +20 Ds 1/6.3 cm = +16 Ds1/6.3 cm = +16 Ds
- 9. Find the Option with equal EVD(IEVD(Istst )) Spectacle magnifier ◦ EVD actual working distance from the spectacle plane ◦ Presbyopes EVD = Equivalent Focal length of add ◦ Non presbyopes (Accommodation should be estimated) EVD = reciprocal of add power + the accommodation ◦ Patient 1 (Hari) +20Ds (EVP) needed to read 0.8 M ( Acco. +8.0 Ds) Required add = +20.00 – (+8.0) = +12.00 Ds will allow him to work at 5 cm (EVD) ◦ Patient 2 (Ram Bdr.) +16 Ds (EVP) needed to read 0.8 m (Acco. +0.50 Ds) Required Add = +16 – (+0.50 ) = +15.50 Ds , EVD = 6.3 cm
- 10. Find the Option equal EVD (IIFind the Option equal EVD (IIndnd )) Hand Held Magnifier ◦ As a Collimator (Option I) Reading material is in focal length I.e image at infinity Retinal image size is independent of separation between eye to magnifier Field of View (FoV)= A (EVD/Z) ◦ Where, A = Lens diameter , Z = the eye to magnifier distance EVD = The focal length of the magnifier ◦ Presbyopes need to wear distance correction ◦ young patient need to relax accommodation As a collimator ◦ A +4.00 Ds magnifier has an EVD of 25 cm ◦ A +10.0 Ds magnifier has an EVD of 10 cm ◦ A +20 Ds magnifier has an EVD of 5 cm
- 11. Find the OptionFind the Option (II(IIndnd )) Hand Held magnifier ◦ Non collimated system ◦ Combined with an add or Accommodation. ◦ EVD = Eye to image distance/ ER = Focal length of add/ ER (Variable) ◦ Eg: Patient is wearing add + 4.00 Ds, with +20.00 Ds HHM ◦ Patient need to adjust the magnifier and object such that image is at 25 cm (Add =4.00 Ds)
- 12. Find the Option (IIFind the Option (IIndnd )) Image alwz at 25 cm, but the ER/EVD varies according to separation of the two lenses HHM toHHM to eyeeye HHM toHHM to focalfocal plane ofplane of addadd (Image)(Image) ERER EVDEVD EVPEVP ObjectObject to HHMto HHM 25 cm25 cm 0cm0cm 1x1x 25 cm25 cm 4.00 Ds4.00 Ds 0 cm0 cm 20cm20cm 5 cm5 cm 2x2x 12.5 cm12.5 cm 8.00 Ds8.00 Ds 2.50 cm2.50 cm 15cm15cm 10cm10cm 3x3x 8.33 cm8.33 cm 12.00 Ds12.00 Ds 3.33 cm3.33 cm 10cm10cm 15 cm15 cm 4x4x 6.25 cm6.25 cm 16.0 Ds16.0 Ds 3.75 cm3.75 cm 5cm5cm 20 cm20 cm 5x5x 5.00 cm5.00 cm 20.00 Ds20.00 Ds 4.00 cm4.00 cm 0cm0cm 25 cm25 cm 6x6x 4.17 cm4.17 cm 24.00 Ds24.00 Ds 4.17 cm4.17 cm
- 13. Find the OptionFind the Option (II(IIndnd )) Operational rules for Hand Held Magnifier (HHM) ◦ If HHM is separated from spectacle ◦ By one magnifier focal length Better resolution will obtain either from distance/Add (no matter) ◦ By more than one magnifier focal length Need to view thru distance portion than add ◦ By less than one magnifier focal length Need to view thru add (or use accommodation) ◦ Held against the spectacle No matter either near / Add But object must be held relatively close to face
- 14. Find the Option equalFind the Option equal EVD(IIIEVD(IIIndnd )) Stand magnifier ◦ Object is fixed / Image distance is fixed ◦ Enlargement ratio (ER) is also fixed ◦ EVD = Eye to image distance./ ER ◦ Eye to image distance Depends upon where the patient’s eye is located Eye to lens distance + lens to image distance
- 15. Calculation of ER in StandCalculation of ER in Stand MagnifierMagnifier Measure ◦ Equivalent power of lens (Fe) & Image distance (l’) ◦ Image vergence (L’) = 1/l’ ◦ ER = L’-Fe / L’ (L’ –ve signifies divergence) ◦ Eg: If, Fe = +20 Ds, l’ = 20cm , L’ = -5 Ds ER = -5 - (+20) / -5 = -25/-5 = 5 x ◦ If, Fe = + 12.50 Ds, l’ = 40 cm , L’ = -2.50 Ds ER = -2.5 – 12.50 /-2.5 = -15/-2.5 = 6 x
- 16. Calculation of EVDCalculation of EVD EVD = eye to image distance/ER Eg: Fe = +20 Ds, l’ = 20 cm below lens surface L = - 5 Ds, EVP = 25/5 = 5x 1. If eye is 5cm from magnifier, Viewing distance = 5 + 20 = 25 cm (eye to image distance) EVD = 25 cm /5 = 5 cm 2. If eye to magnifier is 20 cm, Eye to image = 20 + 20 = 40 cm EVD = 40 cm / 5 (for same stand magnifier EVD will differ from eye to magnifier separation)
- 17. Find the Option equal EVD IVFind the Option equal EVD IVthth Telemicroscope ◦ Distance telescope with a near vision cap ◦ Object is at focal plane of near cap ◦ Cap acts as collimator – image at infinity
- 18. EVD = focal length of cap/EVD = focal length of cap/ telescope magnification (ER)telescope magnification (ER) ◦ Eg: a 3x telescope with a +4.00 Ds cap Cap give EVD of 25 cm (its focal length) But for Telescope with 3x ER EVD = 25 cm/ 3 = 8.33 cm
- 19. Find the option with equal EVD VFind the option with equal EVD Vthth Video-Magnifiers ◦ ER is given ◦ Viewing distance = eye to screen distance = amount of reading add (presbyopes) EVD = fig Actual viewing distance / ER Eg: CCTV enlarges by 10 x , ◦ ER = 10 x , Viewing distance = 50 cm ◦ EVD = 50 cm / 10 = 5 cm
- 20. ConclusionConclusion Manufacturer give unreliable information on their magnifiers : don’t trust them Inappropriate & misleading definition of Magnification ◦ Calculations will be wrong , trial will be tedious WHY to stick to EVD ◦ Reliable ◦ Reproducible No use of misleading magnification
- 21. Requirements for EVD calculationRequirements for EVD calculation Equivalent power of magnifier Checking the image distance for Stand magnifier (Multac factor???)
- 22. ProblemsProblems A patient of 20 yrs has BNA 4M at 20 cm (Acc: 8 Ds) and TNA of 0.5 M – Ajit A patient is 60 yrs has BNA of 3 M at 25 cm and his TNA is 0.8 M - Jevanand A patient is 45 yrs has BNA of 6M at 40 cm (with +2.50 Ds) and his TNA is 1 M – Gopal A patient is 30 yrs has BNA 5M at 30 cm (Acc: 6 Ds) and TNA of 0.6 M – Dipendra A patient of 10 yrs has BNA 8M at 50 cm (Acc: 10 Ds) and TNA of 2M- Sanjay A patient is 80 yrs has BNA of 4 M at 25 cm and his TNA is 1 M - Nabin •Find all the possible options of optical device/ Electronic devices matching EVD?