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－　　－31Published PaperStudy on Applicability of Mean Luminance by Image Photometry as Spatial Brightness IndexMika KATO, Megumi NUMAJIRI, Hideki YAMAGUCHI, Wataru IWAI, Katsuhiko SAKATA, Naoyuki SUZUKI, Naoya HARA and Nozomu YOSHIZWASpatial brightness is the impression of brightness perceived when one observes a certain space, and it is one of the factors in controlling the comfort of a space. In previous studies, Kato clarified the weighted mean luminance that reflects gaze frequency and the decrease in impression due to uneven distribution of luminance distribution. There are many studies on spatial brightness other than Kato's research, but the common index has not been defined yet. Although there are many similar viewpoints between indices, the presence of multiple indices has caused confusion about which index should be used by designers. Therefore, we thought that it was important to organize the explanatory variables between the researches, and the researchers of space brightness in Japan cooperated and set up a committee at the Architectural Institute of Japan and discussed. This paper is a report of an experiment performed by the committee.This paper aims to clarify the applicable range of arithmetic and geometric mean luminance for estimating spatial brightness, compared with the interpretability of the arithmetic mean luminance of walls and a ceiling.The focus of this paper is on mean luminance, which has the strongest effect on the impression of spatial brightness. There are two types of mean luminance: arithmetic mean luminance and geometric mean luminance. As is known by the Weber–Fechner law, it is said that human perception has a high correlation with a logarithmic scale, and there is a view that the geometric mean luminance is appropriate. On the other hand, the illuminance that has been used for a long time in lighting design is an arithmetic mean value, although it is weighted by cosine. The point at issue is which mean luminance is appropriate for explaining spatial brightness.A subjective experiment was conducted in the mock-up of general offices with four kinds of luminaires: ceiling lights, downlights, wall-washer lights and pendant lights. The results showed that the arithmetic mean luminance of the 180- and 360-degree visual fields had a higher correlation with the impression of spatial brightness than the geometric mean luminance of the same visual field. This arithmetic mean indicates the interpretability of the spatial luminance estimate. This result will encourage the use of arithmetic mean brightness as an explanatory variable of spatial brightness. Other variables will continue to be considered based on experimental data.Keywords: Spatial brightness, Arithmetical mean luminance, Geometric mean luminance,空間の明るさ指標としての画像測光による平均輝度の適用性の検討　空間の明るさは，特定の空間を観察したときに知覚される明るさに関する印象であり，空間の快適さを制御する要因の1つである．加藤はこれまでの研究で，視線頻度を反映させた重み付け平均輝度と，輝度分布の偏在による印象の低下の影響を明らかにしてきた．加藤の研究以外にも空間の明るさに関する多くの研究があるが，共通の指標はまだ定義されていない．指標同士の類似視点は多いが，複数の指標があることで，設計者はどの指標を使うべきか混乱が起こっている．そこで，研究相互の説明変数の整理が重要と考え，日本にいる空間の明るさの研究者が協力して，建築学会に委員会を設置し議論を行ってきた．本論文はその委員会で行った実験の報告である．　本論文の検討対象は，空間の明るさの印象に最も強い影響がある平均輝度についてである．平均輝度には，算術平均輝度と幾何平均輝度の２種類がある．ウェーバー・フェヒナーの法則で知られるように，人の感覚は対数スケールとの親和性が高いと言われており，幾何平均輝度が適切との考え方がある．一方で，照明設計に長い間使用されている照度は，コサインの重み付けはあるが，算術平均値である．既存の研究は，どちらの平均輝度を用いて説明するかという点に相違がある．　実験は，4種類の照明器具（シーリングライト，ダウンライト，ウォールウォッシャー，ペンダント）を備えた一般オフィスの実大空間で実施した．結果は，180と360度いずれの視野範囲においても，算術平均輝度は幾何平均輝度よりも高い説明力を持つことを示した．この結果は，算術平均輝度を空間の明るさの説明変数として利用することを後押しするだろう．他の変数についても引き続き実験データに基づく検討が行われていく予定である．キーワード：空間の明るさ，算術平均輝度，幾何平均輝度，Journal（掲載誌）Transactions of AIJ. Journal of environmental engineering, Vol. 84 No. 766, 1059-1066, Dec., 2019