Gloom is a low level of light which is so dim that there are physiological and psychological effects. Human vision at this level becomes monochrome and has lessened clarity, making any environment more unpleasant to be in.
Optical and psychological effects
Light conditions are considered to be gloomy when the level of light in an environment is too low for the proper function of cone cells occurs, and colour vision is lost  In a study by Rothwell and Campbell, light levels described as "gloomy" fell between 28 and 3.6 cd/m^-2.
Low light and lack of color of this sort is associated with depression and lethargy. This association was made as far back as the 2nd century by the ancient Greek physician, Aretaeus of Cappadocia, who said, "Lethargics are to be laid in the light and exposed to the rays of the sun, for the disease is gloom." Also, weaker electrical activity is found in the retinas of depressed people, which gives them poor visual contrast so that they see the world in gray, depressing hues. The naturally weak daylight during winter at extreme latitudes causes Seasonal affective disorder. A solarium or other source of bright light may be used as light therapy to treat such a condition.
Architecture and ergonomics
Where artificial lighting is used, this has to be sufficient to not only illuminate the task area, but also provide sufficient background lighting to avoid a sensation of gloominess which has a negative effect on efficiency. If the task is challenging, such as playing cricket, reaction times are found to increase significantly when the illumination declines to the gloom level.
In architecture, the level of lighting affects whether a building is considered to be unappealing. If there is little or no sunlight or view of the outdoor surroundings from within, then this will tend to make the building seem "gloomy". As seen from the exterior, an interior which is brighter than the surrounding light level may cause the overall building to seem gloomy because the normal cues and contrasts have been upset.
In the arts, a gloomy landscape or setting may be used to illustrate themes such as melancholy or poverty. Horace Walpole coined the term gloomth to describe the ambiance of great ancient buildings which he recreated in the Gothic revival of his house, Strawberry Hill, and novel, The Castle of Otranto. Characters which exemplify a gloomy outlook include Eeyore, Marvin and Old Man Gloom. The catchphrase "doom and gloom", which is commonly used to express extreme pessimism, was popularised by the movie Finian's Rainbow in which the leprechaun Og (Tommy Steele) uses it repeatedly.
Gloomy conditions may arise when low cloud cover forms a continuous overcast. This occurs annually in Southern California, where it is known as June Gloom. Anticyclones may generate gloom-like conditions if they remain stationary, causing a haze and layer of stratocumulus clouds. These tend to occur in temperate winter at the middle latitudes or over an extended period in subtropical regions.
- Susan E. Rothwell; Fergus W. Campbell (1987), "The physiological basis for the sensation of gloom", Ophthalmic and Physiological Optics, 7 (2): 161–163, doi:10.1111/j.1475-1313.1987.tb01014.x
- Tiiler, D; Veltch, J (1995). "Perceived room brightness: Pilot Study on the effect of luminance distribution" (PDF). Lighting Research & Technology. 27: 93–101.
- John M. Eagles (2003), "Seasonal affective disorder", British Journal of Psychiatry, 182: 174–176, doi:10.1192/bjp.182.2.174
- Hannah Devlin (July 21, 2010), "The world truly a grey place for the depressed", Evening Herald
- Emanuel Bubla; Elena Kerna; Dieter Eberta; Michael Bachab; Ludger Tebartz van Elsta (15 July 2010), "Seeing Gray When Feeling Blue? Depression Can Be Measured in the Eye of the Diseased", Biological Psychiatry, 68 (2): 205–208, doi:10.1016/j.biopsych.2010.02.009, PMID 20359698
- AJ Shepherd; WG Julian; AT Purcell (1989), "Gloom as a psychophysical phenomenon", Lighting Research and Technology, 21 (3): 89–97, doi:10.1177/096032718902100301
- Hubert Claude Weston (1949), Sight, light, and efficiency, H. K. Lewis
- AJ Shepherd; WG Julian; AT Purcell (1992), "Measuring appearance: Parameters indicated from gloom studies", Lighting Research and Technology, 24 (4): 203–214, doi:10.1177/096032719202400404
- Fergus W. Campbell; Susan E. Rothwell; Michael J. Perry (19 Dec 2007), "Bad light stops play", Ophthalmic and Physiological Optics, 7 (2): 165–167
- W. M. C. Lam; C. H. Ripman (1992), Perception and lighting as formgivers for architecture, Van Nostrand Reinhold, pp. 24, 54, ISBN 978-0-442-01117-8
- HD Rodee (1977), "The "Dreary Landscape" as a Background for Scenes of Rural Poverty in Victorian Paintings", Art Journal, 36 (4): 307–313, JSTOR 776087
- Hugh Walpole, The Old Ladies
- The Oxford dictionary of allusions, 2001, p. 299
- Darren Harris-Fain (2002), British fantasy and science-fiction writers since 1960, p. 5
- Jennifer Dewey; Jeanie Puleston Fleming, Zozobra!: The Story of Old Man Gloom
- Eric Partridge; Paul Beale (1986), A dictionary of catch phrases, p. 115
- Allaby, Michael (2002). "Anticyclonic gloom". Encyclopedia of Weather and Climate. 1. p. 34. ISBN 978-0-8160-4071-1.
- McIlveen, J. F. Robin (1998). Fundamentals of Weather and Climate. Routledge. p. 393. ISBN 978-0-7487-4079-6.