إشعاعية

تعرف الإشعاعية (إنگليزية: Radiance) على أنها كمية الطاقة الضوئية الصادرة في الثانية من 1 سم² من سطح مصدر التي تنتقل في اتجاه معين ، بشكل عمودي على سطح المصدر في اتجاه الانتقال لكل واحدة للزاوية الصلبة. واحدة الإشعاعية في النظام الدولي للوحدات هي واط لكل متر مربع لكل ستراديان (W·sr^-1·m^-2)، حيث أن المساحة (متر مربع) تكون عمودية على اتجاه انتقال الضوء.

إن توزع الضوء في الفراغ هو تابع للموقع والإتجاه. مثلاً، خذ مصباح جيب يطلق حزمة ضيقة من الضوء في غرفة مظلمة، يجب معرفة مكان المصباح والاتجاه الذي يضيء فيه حتى يمكننا وصف المصباح. هذا يعني أن مساحة صغيرة تطل على مصدر الضوء من الأمام تتلقى كمية أكبر من الطاقة بالمقارنة مع مساحة تكون شبه مماسة للضوء. بشكل مماثل فإن الإشعاعية هي تابع للمكان والاتجاه (مصباح الجيب هو أفضل مثال عليه حيث من الممكن تحريك المصباح في أماكن مختلفة والإشارة بالضوء إلى اتجاهات مختلفة).

من أهم خصائص الإشعاعية أنها تكون ثابتة القيمة على الخط المستقيم ، وهذه الخاصية تـُستخدم كثيرا في تطبيقات البصريات . ويمكن بواسطتها معرفة شدة تألق الشمس والنجوم . يكون من المناسب افتراض أن الضوء لا يتأثر بالوسط الذي ينتشر فيه (أي كأن الضوء ينتشر في الفراغ). بين نقطتين P₁ و P₂ تكون قيمة الإشعاعية التي تغادر النقطة P₁ في اتجاه P₂ مساوياً لقيمة الإشعاعية التي تصل إلى النقطة P₂ من الاتجاه P₁.

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 الحساب

يتم حساب الإشعاعية باستخدام العلاقة التالية:

$L={\frac {\mathrm {d} ^{2}\Phi }{\mathrm {d} A\,\mathrm {d} {\Omega }\cos \theta }}\approx {\frac {\Phi }{\Omega A\cos \theta }}$

حيث

التقريب يكون صالحاً لقيم صغيرة للزواية Ω،

L هو الإشعاعية (W·m^-2·sr^-1)،

Φ هو التدفق الإشعاعي للطاقة(W)،

θ هي الزاويةبين العمودي على السطح والإتجاه المعين،

A هي مساحة المنبع الضوئي(m²)،

${\Omega }$ هي الزاوية الصلبة (sr).

 الوحدات الدولية لقياس الإشعاع

الوحدات الدولية لقياس الإشعاع

الكمية		الوحدة		الأبعاد	ملاحظات
الاسم	الرمز	الاسم	الرمز	الرمز
طاقة إشعاعية	Qe	جول	J	M⋅L2⋅T−2	طاقة الإشعاع الكهرومغناطيسي.
كثافة الطاقة الإشعاعية	we	joule per cubic metre	J/m3	M⋅L−1⋅T−2	الطاقة الإشعاعية لكل وحدة حجم.
التدفق الإشعاعي	Φe	واط	W = J/s	M⋅L2⋅T−3	Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power".
تدفق طيفي	Φe,ν	واط لكل هرتس	W/Hz	M⋅L2⋅T−2	Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅nm−1.
	Φe,λ	واط للمتر	W/m	M⋅L⋅T−3
الشدة الإشعاعية	Ie,Ω	watt per steradian	W/sr	M⋅L2⋅T−3	Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a directional quantity.
الشدة الطيفية	Ie,Ω,ν	watt per steradian per hertz	W⋅sr−1⋅Hz−1	M⋅L2⋅T−2	Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅nm−1. This is a directional quantity.
	Ie,Ω,λ	watt per steradian per metre	W⋅sr−1⋅m−1	M⋅L⋅T−3
إشعاعية	Le,Ω	watt per steradian per square metre	W⋅sr−1⋅m−2	M⋅T−3	Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. This is a directional quantity. This is sometimes also confusingly called "intensity".
الإشعاعية الطيفية	Le,Ω,ν	watt per steradian per square metre per hertz	W⋅sr−1⋅m−2⋅Hz−1	M⋅T−2	Radiance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅m−2⋅nm−1. This is a directional quantity. This is sometimes also confusingly called "spectral intensity".
	Le,Ω,λ	watt per steradian per square metre, per metre	W⋅sr−1⋅m−3	M⋅L−1⋅T−3
التشعيع كثافة التدفق	Ee	واط للمتر المربع	W/m2	M⋅T−3	Radiant flux received by a surface per unit area. This is sometimes also confusingly called "intensity".
التشعيع الطيفي كثافة التدفق الطيفي	Ee,ν	watt per square metre per hertz	W⋅m−2⋅Hz−1	M⋅T−2	Irradiance of a surface per unit frequency or wavelength. This is sometimes also confusingly called "spectral intensity". Non-SI units of spectral flux density include jansky (1 Jy = 10−26 W⋅m−2⋅Hz−1) and solar flux unit (1 sfu = 10−22 W⋅m−2⋅Hz−1 = 104 Jy).
	Ee,λ	watt per square metre, per metre	W/m³	M⋅L−1⋅T−3
كثافة التدفق الإشعاعي Radiosity	Je	watt per square metre	W/m²	M⋅T−3	Radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area. This is sometimes also confusingly called "intensity".
كثافة التدفق الطيفي Spectral Radiosity	Je,ν	watt per square metre per hertz	W⋅m−2⋅Hz−1	M⋅T−2	Radiosity of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. This is sometimes also confusingly called "spectral intensity".
	Je,λ	watt per square metre, per metre	W/m³	M⋅L−1⋅T−3
Radiant exitance	Me	watt per square metre	W/m2	M⋅T−3	Radiant flux emitted by a surface per unit area. This is the emitted component of radiosity. "Radiant emittance" is an old term for this quantity. This is sometimes also confusingly called "intensity".
Spectral exitance	Me,ν	watt per square metre per hertz	W⋅m−2⋅Hz−1	M⋅T−2	Radiant exitance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. "Spectral emittance" is an old term for this quantity. This is sometimes also confusingly called "spectral intensity".
	Me,λ	watt per square metre, per metre	W/m3	M⋅L−1⋅T−3
Radiant exposure	He	joule per square metre	J/m2	M⋅T−2	Radiant energy received by a surface per unit area, or equivalently irradiance of a surface integrated over time of irradiation. This is sometimes also called "radiant fluence".
Spectral exposure	He,ν	joule per square metre per hertz	J⋅m−2⋅Hz−1	M⋅T−1	Radiant exposure of a surface per unit frequency or wavelength. The latter is commonly measured in J⋅m−2⋅nm−1. This is sometimes also called "spectral fluence".
	He,λ	joule per square metre, per metre	J/m³	M⋅L−1⋅T−2
Hemispherical emissivity	ε	N/A		1	Radiant exitance of a surface, divided by that of a black body at the same temperature as that surface.
Spectral hemispherical emissivity	εν  or ελ	N/A		1	Spectral exitance of a surface, divided by that of a black body at the same temperature as that surface.
Directional emissivity	εΩ	N/A		1	Radiance emitted by a surface, divided by that emitted by a black body at the same temperature as that surface.
Spectral directional emissivity	εΩ,ν  or εΩ,λ	N/A		1	Spectral radiance emitted by a surface, divided by that of a black body at the same temperature as that surface.
Hemispherical absorptance	A	N/A		1	Radiant flux absorbed by a surface, divided by that received by that surface. This should not be confused with "absorbance".
Spectral hemispherical absorptance	Aν  or Aλ	N/A		1	Spectral flux absorbed by a surface, divided by that received by that surface. This should not be confused with "spectral absorbance".
Directional absorptance	AΩ	N/A		1	Radiance absorbed by a surface, divided by the radiance incident onto that surface. This should not be confused with "absorbance".
Spectral directional absorptance	AΩ,ν  or AΩ,λ	N/A		1	Spectral radiance absorbed by a surface, divided by the spectral radiance incident onto that surface. This should not be confused with "spectral absorbance".
Hemispherical reflectance	R	N/A		1	Radiant flux reflected by a surface, divided by that received by that surface.
Spectral hemispherical reflectance	Rν  or Rλ	N/A		1	Spectral flux reflected by a surface, divided by that received by that surface.
Directional reflectance	RΩ	N/A		1	Radiance reflected by a surface, divided by that received by that surface.
Spectral directional reflectance	RΩ,ν  or RΩ,λ	N/A		1	Spectral radiance reflected by a surface, divided by that received by that surface.
Hemispherical transmittance	T	N/A		1	Radiant flux transmitted by a surface, divided by that received by that surface.
Spectral hemispherical transmittance	Tν  or Tλ	N/A		1	Spectral flux transmitted by a surface, divided by that received by that surface.
Directional transmittance	TΩ	N/A		1	Radiance transmitted by a surface, divided by that received by that surface.
Spectral directional transmittance	TΩ,ν  or TΩ,λ	N/A		1	Spectral radiance transmitted by a surface, divided by that received by that surface.
Hemispherical attenuation coefficient	μ	reciprocal metre	m−1	L−1	Radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume.
Spectral hemispherical attenuation coefficient	μν  or μλ	reciprocal metre	m−1	L−1	Spectral radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume.
Directional attenuation coefficient	μΩ	reciprocal metre	m−1	L−1	Radiance absorbed and scattered by a volume per unit length, divided by that received by that volume.
Spectral directional attenuation coefficient	μΩ,ν  or μΩ,λ	reciprocal metre	m−1	L−1	Spectral radiance absorbed and scattered by a volume per unit length, divided by that received by that volume.
See also: SI • Radiometry • Photometry

 انظر أيضاً

• Étendue
• Light field
• Sakuma–Hattori equation
• Wien displacement law

 وصلات خارجية

• International Lighting in Controlled Environments Workshop