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Human Factors Engineering

Dr. Osama Al Meanazel

Lecture 4

(Deigning Visual Displays)

September 26, 2013

“The Efficacy of a Medical Virtual Reality Simulator for Training Phlebotomy” by Mark W. Scerbo, James P. Bliss, Elizabeth A. Schmidt, and Sommer N. Thompson

Objective: The present study compared the effectiveness of a virtual reality (VR) simulator for training phlebotomy with that of a more traditional approach using simulated limbs. Background: Phlebotomy, or drawing blood, is one of the most common medical procedures; yet, there are no universal standards for training and assessing performance. The absence of any standards can lead to injuries and inaccurate test results if the procedure is improperly performed. Method: Twenty 3rd-year medical students were trained under one of the two methods and had their performance assessed with a 28-item checklist. Results: The results showed that performance improvements were limited to those who trained with the simulated limbs, and a detailed comparison of the two systems revealed several functional and physical differences that may explain these findings. Conclusion: Participants trained with simulated limbs performed better than those trained with a VR simulator; however, the metrics recorded by the VR system may address some aspects of performance that could eventually prove beneficial. Application: The present study highlights the potential for medical simulators to improve patient safety by enabling trainees to practice procedures on devices instead of patients. Applications of this research include training, performance assessment, and design of simulator systems.

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“+3Gz Acceleration loss of consciousness: Tie course of performance deficits with repeated experience” by Lioyd D. Trip et al.

Objectives: We examine the time course of performance recovery from gravity induced loss of consciousness

(GLOC) and evaluate the utility of exposing participants to repeated bouts of GLOC in promoting recovery time.

Background: A substantial number of accidents among fighter pilots have resulted from episodes of GLOC. U.S. Air

Force doctrine holds that when pilots experience GLOC, impairment lasts for 24 s, in which there are 12 s of

complete unconsciousness and 12 s of confusion. However, there is reason to suspect that performance efficiency

associated with GLOC is degraded well before unconsciousness sets in and that more than 24 s are required for

performance efficiency to return to baseline levels. Additionally, there is a belief that repeated exposure to GLOC will

reduce recovery time. Method: Centrifuge simulators were used to induce GLOC in U.S. Air Force personnel with

math and tracking tasks employed to emulate flight performance. Participants were tested once per week for 4

consecutive weeks. Results: On average, performance deficits appeared 7.44 s prior to the onset of unconsciousness

and persisted for 55.6 s following the GLOC event. Repeated exposure failed to moderate these results. Conclusion:

The temporal course of performance deficits produced by GLOC far exceeds prior estimates. The problem is more

serious than previously envisioned and it is not alleviated by repeated exposure to GLOC. Application: U.S. Air

Force doctrine regarding the severity of GLOC and the utility of repeated exposure to this problem needs to be

revised and these data incorporated into future aircraft auto recovery systems.

Research Settings

(1) Whether it is a descriptive study, experimental research, or evaluation research?

(2) Whether it was conducted in a laboratory, field, or using simulation?

(3) Who the subjects were and how many were used?

(4) What the variables were?

(5) What type of criterion measures were used?

(6) What statistical analyses were conducted.

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Classifications of Visual Displays Information

Static visual displays information • Traffic signs, warning signs, direction signs … etc

Dynamic visual displays information • Speedometer, pressure gauges, thermometer … etc

They Eye

1. Cornea

2. Pupil

3. Lens

4. Retina

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How we see?

1. Light enter the eye

2. Reflected by the cornea and the lens

3. Focus on the retina (Contains two types of receptors rods and cones)

• Rods: Function at low levels of light and can distinguish only between shades of black and

white, not evenly distributed

• Cones: Function at high levels of light and can differentiate between colors, concentrated

at the center

4. Curvature of the lens increases by contraction of the muscles surrounding the

lens

Retina Adaptation

Adaptation is the change in sensitivity to light (Light Dark)

Sensitivity of the visual system increases in the dark (we can see dimmer and dimmer objects)

Two stages to dark adaptation: • Stage 1: Cones adaptation (takes 5 min)

• Stage 2: Rods adaptation (takes 30-35 min)

Dark to light adaptation takes place very fast within a matter of minutes.

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Color Discrimination and Reading

Color Discrimination • People with normal vision: trichromats

• People with complete lack of color vision: monochromats

Most people are color deficient due to red or green receptors.

Reading • Reading goes beyond acuity and letter recognition.

• It involves eye movements which consists of fast jerky movements (saccades) and stationary periods (fixations).

• Information is absorbed during fixations.

Visual Capabilities

Accommodation: the ability of the lens to focus the light rays on the retina. • Near point: closest point to which the eyes can focus

• Far point: farthest point beyond which the eyes can not focus (infinity)

Focal points and distances are expressed in diopters (D).

Near Sightedness (Myopia): People can not see distant objects, their far point is too close.

For sightedness (Hypermania): People can not see near objects, their near point is far away.

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Visual Acuity

Visual Acuity is the ability to discriminate fine detail and depends on the accommodation of eyes.

Most commonly used measure of visual acuity : minimum separable acuity • Refers to the smallest space between the parts of the target that the eye can detect.

Visual Angle and 20/20 Vision

Visual Angle (minutes) = 3438 * H / D

‘Normal’ acuity is usually taken as 1.0 (VA = 1 min)

A person with 20/20 vision (on Snellen acuity) is assumed to be of the ability to resolve a target of 1 min of arc at 20 ft

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Snellen Acuity

Photometry: Basic Concepts

Source

Reflective surface

Observer

Luminous flux

- lumens

Illuminance

- lux

- foot-candles

Luminance

- foot-lamberts

- milli-Lamberts

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Common Definition

Light intensity: the amount of light given off by a source. It is measured in candelas and lumens (4π lumens = 1 candela)

Illumination level: the amount of light falling on a surface area measured in foot-candles (fc) or lux (lumen/m2) • 1 lux = 10.76 fc

Luminance: measure of light reflecting from a surface. The units for luminance are foot-lamberts or candela/m2

Reflectance: the ratio of luminance to illumination. • Black surfaces has Zero reflectance • White surfaces has One reflectance

Factors affecting Visual Acuity

1. Luminance level: Visual acuity improves with increasing levels of light or background luminance and then levels off. At high levels of illumination “cones“ are activated

2. Contrast: Improved contrast increases discriminability. • Luminous Contrast = (Lmax - Lmin ) / Lmax • Contrast (or luminance) ratio = Lmax / Lmin

3. Target Motion: Movement of the target or the observer decreases visual acuity • Static acuity: When both target and observer are stationary • Dynamic acuity: When either one of them is moving

4. Age: Visual Acuity declines with age (after 40)

5. Training: Training does improve visual acuity

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Text: Hardcopy

Important things to be considered when Presenting Alphanumeric Text (Hardcopy)

• Visibility: Quality of characters / symbol that makes it visible from its

surroundings (detectability)

• Legibility: Attribute of alphanumeric characters that makes each one of them identifiable from others (discriminability)

• Readability: A quality that makes possible the recognition of the informational content when it is presented in groups

Typography and Stroke Width-Height

Typography: refers to the form / features of the characters Example: using the times roman font over script font is preferred during

• unfavorable viewing conditions

• critical information is to be displayed

• large viewing distance

• people have poor vision

Stroke Width – Height: refers to ratio of the thickness of the stroke to the height of the letter.

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Stroke Width

Black letter on white background – preferred

White letters on black background – irradiation (letters appear to speared)

Black on white letters should be thicker than white on black

Width-to-Height Ratio

Thickness (stroke width-Height Ratio) Black on White – 1:6 to 1:8 White on Black – 1:8 to 1:10 Luminous letters – 1:12 to 1:20

Width-to-Height Ration • The relationship between the width and the height of the alphanumeric character

is referred to as the width-height ration • Most commonly used ration is 3:5

Styles of Type • There are thousands of typefaces/fonts. The most commonly used font is the

Roman. Other types are Gothic, script and Block-Letter

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Stroke Width-to-Height Ratio

Size

Size of Alphanumeric Characters

The size of the letter is measured in points. • One point (pt) = 0.25 mm.

Most commonly used font size for close reading varies 9 pt - 12 pt

What would be the visual angle in minutes subtended by a letter of size 10 pt at the eye when viewing it from 50 cm? 10 pt = 10 * 0.25 = 2.5 mm H = 2.5 mm; D = 500 mm; VA = 3438 * H/D = 3438 * 2.5 / 500 = 17.19 minutes

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Distance Reading

In order that the legibility of characters is retained as we move away from it, it is necessary to increase the size of the characters so that they subtend the same visual angle

NBS formula for stroke width of letters WS= 1.45 * S * d * 10-5 HL = WS / R WS : Stroke Width S : Denominator of snellen acuity score (Snellen acuity = 20/20, S=20) D : Reading distance HL : Letter height R : stroke width-to-height ratio of the font expressed in decimal proportion (R= 0.20 for a ratio of 1:5)

Recommended heights of alphanumeric characters for critical and non-critical use under low and high illumination are shown in the following table

Distance Reading

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Case and Layout

Case • IT IS NOT EASY TO READ EVERYTHING IN UPPERCASE !!!

• It is not easy to read everything in uppercase

Layout • Two factors to be considered: inter-letter spacing and inter-line spacing

• The spacing between letter in the text can be varied. Moriary counted the words read by subjects from two sales brochure.

• T h e s p a c i n g b e t w e e n l e t t e r i n t h e t e x t c a n b e v a r i e d . M o r i a r y c o u n t e d t h e w o r d s r e a d b y s u b j e c t s f r o m t w o s a l e s b r o c h u r e .

Layout

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Screen Design Issues for Computer Displays

Various guidelines have been developed: Smith and Mosier, Browns, etc.

• Density (local density) • Grouping (perceptual grouping) • Complexity (extent to which arrangement follows a predictable scheme) • Highlighting

Graphical Representation of Data • Symbols • Coding

Graphic Representation and Symbols

Graphic Representation • Graphic Representations of Text

• Graphic Representation of Data.

Symbols • Comparison of Symbolic and Verbal Signs.

• Objectives of Symbolic Coding Systems.

• Criteria for Selecting Coding Symbols.

• Recognition, Matching, Preference and Opinion.

• Perceptual Principles of Symbolic Design.

• Standardization of Symbolic Displays.

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Graphic Representation of Data

Perceptual Principles of Symbolic Design

The percentages below the symbols are the percentages of correct recognition as follows: O = Original Test; R = Recall 1 week later

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In-Class Exercise

In-Class Exercise