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diff --git a/src/ipa/raspberrypi/controller/lux_status.h b/src/ipa/raspberrypi/controller/lux_status.h
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+++ b/src/ipa/raspberrypi/controller/lux_status.h
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+/* SPDX-License-Identifier: BSD-2-Clause */
+/*
+ * Copyright (C) 2019, Raspberry Pi (Trading) Limited
+ *
+ * lux_status.h - Lux control algorithm status
+ */
+#pragma once
+
+// The "lux" algorithm looks at the (AGC) histogram statistics of the frame and
+// estimates the current lux level of the scene. It does this by a simple ratio
+// calculation comparing to a reference image that was taken in known conditions
+// with known statistics and a properly measured lux level. There is a slight
+// problem with aperture, in that it may be variable without the system knowing
+// or being aware of it. In this case an external application may set a
+// "current_aperture" value if it wishes, which would be used in place of the
+// (presumably meaningless) value in the image metadata.
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct LuxStatus {
+	double lux;
+	double aperture;
+};
+
+#ifdef __cplusplus
+}
+#endif