Productivity Management Imperative for USA - McKinsey. Returning US productivity to its long-term trend of 2.2 percent annual growth would add $10 trillion in cumulative GDP over the next ten years (2023 - 2030).
INTRODUCTION TO MODERN INDUSTRIAL ENGINEERING. E-Book FREE Download.
6. PHYSICAL FACTORS OF THE WORK ENVIRONMENT
Lesson 220 of IEKC Industrial Engineering FREE ONLINE Course.
Chapter 5. WORKPLACE, JOB & PRODUCT DESIGN (Abridged with Comments)
OHTA STUDENT MANUAL
Ergonomics Essentials
April 2009
This manual was originally developed by BP and University of Wollongong. The Occupational Hygiene Training Association Ltd would like to acknowledge the contribution of these organisations in funding and developing the material and is grateful for their permission to use and modify it.
Supported by OHTA, IOHA
This work is licensed under a Creative Commons Attribution-No Derivative Works Licence
SENSING THE ENVIRONMENT
Information is conveyed to the brain through sense organs such as the eyes, ears and nose.
The stimulus has to be strong enough for the senses to detect before a person can be aware of any stimulation from the environment. The ‘absolute threshold’ marks the difference between being aware and not being aware of a stimulus and this may vary at different times and under different conditions.
A second threshold, termed the ‘difference threshold’, refers to detectable differences between two stimuli that can be observed by an individual. This discrimination between the stimuli is also termed the just noticeable difference (JND). The key question is, can the user discriminate one code/stimuli from another, and the JND needs to be established to determine this.
People’s senses adapt to various stimuli in different situations. If the stimulus is constant and familiar the sense organs can become insensitive to it.
6.1 VISION & LIGHTING
6.1.1 The Eye & Visual Capabilities
a) Structure of the Eye
The eye operates like a camera catching and refracting light and converting it to a picture. The light passes through the cornea (the clear structure at the front of the eye) and through the pupil (the small
aperture). The iris (the coloured part of the eye) controls the aperture of the pupil, allowing variable amounts of light to pass through.
There are two muscles that control the iris and hence the size of the pupil. The iris constricts to make the pupil smaller in the following situations:
• High light levels
• Looking at near objects
• When the eye is irritated
The iris dilates, making the pupil larger in the following situations:
• Low light levels
• When you are alarmed or in pain
The light is focused by the crystalline lens onto the retina. The retina consists of a layer of nerves fibres, and the information it receives is transferred to the brain via the optic nerve. Only objects focused on a
tiny area on the posterior surface of the eye, the fovea, are seen clearly. The rest of the retina, known as the peripheral retina, is used for peripheral or side vision. Normally the eye moves about very rapidly
so that each part of the visual field falls on the fovea in turn, allowing the brain to gradually build up a sharp picture of the whole surrounding.
The two types of cells in the retina that collect the light and transfer the information are the rods and cones.
• Rods - Rods are extremely sensitive to light, but they cannot detect shape or colour. Rods are only in the peripheral retina.
• Cones – These are most dense in the fovea, and detect blue light, green light and red light. There are also cones in the peripheral retina, but they decrease in density as the distance increases from the fovea.
While it is important for the whole eye to be healthy, the iris, pupil and retina play a large part in the collection and interpretation of light.
b) Visual Acuity
Visual acuity refers to the detection and discrimination of fine detail in visual stimuli. Visual acuity can be assessed used eye charts such as the Snellen Chart. A person with normal acuity can read the 6th line of the chart at a distance of 6 metres (hence the expression 6/6 vision). In the UK and USA the measure is 20th line of test chart at a 20 ft distance (20/20 vision).
To focus on objects at different distances the curvature of the lens of the eye is adjusted using the ciliary muscle, and this process is known as accommodation. The ciliary muscle is relaxed for long vision and
contracts for near vision.
The ability to focus changes with age as the speed and range of accommodation reduces. With the ageing process the lens loses its elasticity, making focusing on near objects difficult. A person’s ‘near point’ or the closest distance from the eye at which one can focus increases with age – from about 11cm at age 20 to about 50cm at age 50 – thus reducing the acuteness of the vision (ie: visual acuity).
To improve visual acuity one can increase illumination and/or increase contrast. The extra light on the work can improve visual acuity as it causes the pupil to contract and reduces the aperture of the lens.
Glasses can also be prescribed to counteract visual problems.
6.1.2 Lighting For Work
a) Illuminance & Luminance
Lighting levels are usually measured in terms of the illuminance, which is the amount of light falling on a surface (luminous flux incident on a surface per unit area). It is generally measured with a light meter lying on or against the work surface, such as the horizontal work surface of a desk, or the vertical work surface of a whiteboard etc. The unit for illuminance is lumens per square metre, or lux (lx).
The maximum illuminance from a given source is obtained by moving close to, and directly under the source. As well as light falling on surfaces (illuminance), light is emitted by surfaces and objects (luminance). This light can be measured by a light meter or photometer pointed at the surface and this is measured in candela per metre squared (cd m2). The luminance or brightness of the surface or object will vary according to the intensity of the source light and the reflectivity of the surface.
Luminance = illuminance x reflectivity
Illuminance and luminance readings of a surface allow its reflectivity to be calculated. Work surfaces with a high reflectivity can cause annoying glare.
The International Standards (eg: ISO/CIE 8995 parts 1 & 2) provide advice on the levels on illuminance and other lighting characteristics for different activities undertaken indoors and outdoors. They include guidance for general building areas plus for specific work tasks covering broad ranging workplaces such as: agriculture, manufacturing, education, transport, offices, power stations etc.
b) Luminaires
The term used for lighting installations or light fittings is ‘luminaires’ and these are fitted with various lamp types. Common lamp types are incandescent (such as conventional incandescent (heated filament) or tungsten halogen lamps) or gas discharge lamps (such as fluorescent lamps).
In determining the amount of light that must fall onto a work surface, it is necessary to distinguish between orientation lighting, normal working lighting and special lighting.
c) Orientation Lighting
Select a light intensity of 10-200 lux for orientation tasks. The minimum required intensity to detect obstacles is 10 lux. A light intensity of 10-200 lux is sufficient where the visual aspect is not critical, such as in corridors of public buildings, or for general activities in storerooms, provided no reading is required. A higher light intensity may be necessary for reading or to prevent excessive differences in brightness between adjoining areas. Where eyes need to adjust rapidly when moving between the areas, such as when driving into tunnels, reduce the differences in brightness.
d) Normal Working Lighting
Select a light intensity of 200-800 lux for normal visual tasks such as reading normal print, operating machines and carrying out assembly tasks. Where the details are small or hard to read, the person is older or has visual difficulties or where there are great contrasts of light such as near windows, more light will be needed.
e) Special Lighting
Select a light intensity of 800 - 3000 lux for special applications. It is sometimes necessary to use desk lighting to compensate for shadows or reflection on the work surface. Intensive activities requiring precision such as visual inspection tasks require much higher illumination levels to distinguish fine detail.
Avoid excessive differences in brightness within the visual field. Reflections, dazzling light and shadows can all cause difficulty in seeing. Use a combination of ambient (general) and localised or task lighting for localised tasks.
f) Designing Lighting Systems
To achieve the best lighting systems in a workplace, each of the following factors must be considered:
• Task – detail, large/small, stationary/moving, contrast
• Viewer – young or old, visual impediments
• Room or area – wide, narrow, high/low
• Environment – clean/dirty, maintenance, surface reflections
• Luminaire – open/closed, upward/downward, beam shape, environment
• Lamp – type, colour, life, lumen output, wattage
• Illuminance level – refer to relevant Standards
• Uniformity – layout, modelling/shadows (ensure lighting levels are uniform in work area – check for shadowing effects)
• Maintenance – of lamps, luminaries, room surfaces, windows
• Daylight – availability, energy conservation, good colour, user well being, glare
(Philip Saks, School of Electrotechnology, Regency Institute of TAFE 2000)
In summary, for any lighting system the aims are:
• For the task to be seen easily – with suitable maintenance illumination, and with no disability glare or veiling reflections
• Promote user comfort – with no discomfort glare and
• Enhance safety – with adequate maintenance illumination levels, no flicker effects and with adequate colour rendering
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